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ucaddin.dll is a Microsoft Office and Skype for Business file used by the Outlook integration for scheduling Skype meetings. If Windows says it is missing or cannot load, repair the matching Office or Skype for Business installation rather than downloading a replacement DLL.
What this file actually does
ucaddin.dll belongs to the Skype for Business Outlook integration, commonly called the Online Meeting Add-in for Skype for Business. It helps classic Outlook create, schedule, and manage Skype meetings.
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It is associated with Skype for Business clients and Microsoft Office or Microsoft 365 Apps installations, including Office-era deployments using Skype for Business 2015, 2016, or related client releases. The Basic client may also install the integration when its architecture matches the rest of Office.
This is not a normal Windows system DLL. Do not copy it to:
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C:WindowsSystem32C:WindowsSysWOW64- The Windows directory
- A game or unrelated application folder
Legitimate installations commonly place it under one of these Office directories:
C:Program FilesMicrosoft OfficeRootOffice16UCAddin.dll
C:Program Files (x86)Microsoft OfficeRootOffice16UCAddin.dll
Older MSI-based Office installations may use an Office15 or Office16 directory. The correct version depends on the installed client, Office build, update level, and system architecture. Microsoft has documented historical versions of the file, but there is no single version, size, or hash that is correct for every computer.
Microsoft describes the client, Office compatibility, installation technologies, and 32-bit and 64-bit variants in its Skype for Business client documentation.
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| Symptom | Most likely cause | Best first action |
|---|---|---|
| “UCAddin.dll not found” when Outlook starts | Damaged or incomplete Office installation | Repair Microsoft 365 or Office |
| Error appears while creating a Skype meeting | Broken Skype Meeting add-in or Outlook integration | Repair Office, then check COM add-ins |
| “Could not be loaded” | Corruption, wrong architecture, or missing dependency | Confirm Office bitness and run Online Repair |
| “Not a valid Win32 application” | 32-bit and 64-bit mismatch or corrupt file | Reinstall the matching client |
| “Not designed to run on Windows” | Damaged or mismatched Office file | Repair or reinstall the parent product |
| Outlook disables the add-in | Outlook detected a crash or performance problem | Repair first, then re-enable the add-in |
| File exists but the error remains | Dependency, registration, or incompatible build problem | Repair the complete Office/Skype installation |
| Error begins after antivirus action | The file may have been quarantined | Verify the path and Microsoft signature before restoring |
A message naming ucaddin.dll does not prove that the DLL itself is the root cause. The actual problem may be a missing dependency, an interrupted Office update, incompatible installation technologies, or an add-in that Outlook disabled.
Fix ucaddin.dll errors in order
1. Repair Microsoft 365 or Office
This is the safest and most likely fix when the file is under an Office directory or the error appears in Outlook.
From Windows Settings
- Close Outlook, Word, Skype for Business, and other Office programs.
- Right-click Start and select Installed apps.
- Find Microsoft 365, Microsoft Office, or the relevant Office installation.
- Select the three-dot menu beside it.
- Choose Modify.
- Select Quick Repair, then choose Repair.
- Restart Windows and test Outlook.
If Quick Repair does not restore the add-in:
- Repeat steps 1 through 5.
- Select Online Repair.
- Follow the prompts until the repair completes.
- Restart Windows.
- Open classic Outlook and try creating a Skype meeting.
Online Repair replaces damaged Office components and is more thorough, but it can take longer and may require Office to be configured again.
From Control Panel
For older installations:
- Press Windows key, type
Control Panel, and open it. - Select Programs > Programs and Features.
- Select Microsoft 365, Microsoft Office, or the relevant Office entry.
- Choose Change.
- Select Online Repair and follow the prompts.
- Restart Windows and test the Skype Meeting integration.
Microsoft documents the Office repair process in its Outlook and Office repair instructions.
The repair worked if Outlook starts without the DLL message and the Skype meeting command becomes available again. If the error remains, continue with the architecture check below instead of copying a DLL manually.
2. Match 32-bit and 64-bit Office
A 32-bit Office installation needs the compatible 32-bit Skype for Business integration. A 64-bit Office installation needs the compatible 64-bit edition. Installing the wrong architecture can produce missing-file, invalid-application, or load errors even when a file is present.
To check Office architecture:
- Open Word or another Office application.
- Select File > Account.
- Select About Word, About Outlook, or the equivalent link.
- Read the information at the end of the window. It identifies whether Office is 32-bit or 64-bit.
- Close the Office application.
If Skype for Business Basic is installed separately, remove and reinstall the edition matching Office. Microsoft provides the supported Skype for Business Basic Client and explains why the architecture must match the other Office applications.
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Do not combine 32-bit Office with a 64-bit add-in package, or 64-bit Office with a 32-bit package. If you are unsure which components belong together, repairing the existing Office installation is safer than installing a standalone client.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errors3. Update or reinstall Skype for Business from Microsoft
If Skype for Business was installed separately from Office, its own installation may be damaged or out of sync with Office.
Microsoft’s supported Skype for Business installation instructions cover Microsoft 365 installations and the Basic client. The installer name may appear as setupskypeforbusinessentryretail.exe; older MSI deployments may use Lyncentry.msi.
Use this sequence:
- Close Outlook and Skype for Business.
- Open Settings > Apps > Installed apps.
- Search for Skype for Business.
- If a Modify or Repair option is available, run it.
- If repair is unavailable or fails, uninstall the matching Skype for Business client.
- Restart Windows.
- Install the correct client from Microsoft.
- Confirm that its 32-bit or 64-bit edition matches Office.
- Start Skype for Business once, then open classic Outlook.
For a Click-to-Run Skype for Business 2016 installation:
- Open Word.
- Select File > Account.
- Select Update Options.
- Choose Update Now.
- Restart Windows after the update completes.
MSI installations use Windows Update or the applicable Microsoft update package instead. Microsoft explains the different update routes in its guide to obtaining Skype for Business 2016 updates.
The fix worked if Skype for Business starts normally and classic Outlook can insert a Skype meeting without crashing.
4. Re-enable the add-in after repairing it
Outlook can disable a COM add-in after repeated crashes or slow startup. Re-enabling it before repairing the underlying installation can cause the problem to return.
In classic Outlook:
- Open Outlook.
- Select File > Options.
- Select Add-ins.
- At the bottom, set Manage to COM Add-ins.
- Select Go.
- Look for the Skype Meeting or Skype for Business add-in.
- Select its checkbox.
- Choose OK.
- Restart Outlook.
Some Outlook versions show disabled items under File > Manage COM Add-ins. Microsoft describes this behavior in its guidance about Outlook add-in warnings.
If the add-in immediately disables itself again, do not repeatedly re-enable it. Repair Office again, then test Outlook with other COM add-ins disabled. A persistent crash may involve an Office build, Outlook profile, another add-in, or a Skype and Teams integration conflict.
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5. Check antivirus quarantine
Security software may quarantine a legitimate Office file, or it may identify an altered file that should not be restored.
- Open your antivirus or Windows Security application.
- Open Protection history, Quarantine, or the equivalent area.
- Look for
UCAddin.dll. - Inspect its original path.
- Restore it only if it came from the legitimate Office directory and the file is digitally signed by Microsoft.
- Run Office Repair afterward.
If the file is unsigned, came from an unexpected directory, or the detection appears credible, do not restore it. Remove and reinstall Office or Skype for Business from Microsoft, then run a complete malware scan.
6. Use DISM and SFC only for broader Windows corruption
ucaddin.dll is normally an Office or Skype for Business file, not a protected Windows component. Therefore, SFC may report no problem even when this DLL is missing.
Use these tools if Windows has other corruption symptoms, system files are failing, or Office Repair cannot complete.
- Right-click Start.
- Select Terminal (Admin) or Command Prompt (Admin).
- Approve the User Account Control prompt.
- Run:
DISM /Online /Cleanup-Image /RestoreHealth
- Wait for it to finish.
- Then run:
sfc /scannow
- Restart Windows.
- Run Office Online Repair again if the DLL error remains.
Microsoft recommends running DISM before SFC in its System File Checker guidance.
Repeating SFC will not rebuild a missing Office add-in. If both commands complete without repairing the problem, return to Office or Skype for Business repair.
When Windows component repair requires manual diagnosis, Outbyte PC Repair can scan for the broader issues that make repeated SFC/DISM attempts difficult to interpret; its free scan shows what it finds, while repair is handled by the full version. Reinstalling Office or Skype for Business remains the definitive fix for this application-owned DLL.
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Device Manager repairs hardware drivers, not the Skype for Business Outlook add-in. Use it only if the computer also has display, audio, network, or other hardware failures that could be affecting Office.
To check for related driver problems:
- Right-click Start.
- Select Device Manager.
- Expand categories such as Display adapters, Sound, video and game controllers, and Network adapters.
- Look for a yellow warning icon.
- Right-click a clearly affected device.
- Select Update driver.
- Choose Search automatically for drivers.
- Restart Windows if a driver is installed.
Do not remove random devices because ucaddin.dll is not a hardware driver. If you are manually comparing driver versions across several devices, Outbyte Driver Updater can scan for outdated drivers; its free scan identifies issues, while installation is handled by the full version. This is optional and does not replace repairing the Office or Skype installation.
8. Consider System Restore or a new profile
Use System Restore only when the failure clearly began after a recent system change and an appropriate restore point exists.
- Press Windows key and search for Create a restore point.
- Open it.
- Select System Restore.
- Choose a restore point from before the problem began.
- Review the affected programs and complete the wizard.
If only one Windows user experiences the problem:
- Repair Office first.
- Create a new Outlook profile through Control Panel > Mail > Show Profiles.
- Choose Add.
- Configure the account.
- Start Outlook with the new profile.
- Test the Skype Meeting add-in.
A profile test helps distinguish a user-specific Outlook problem from a damaged Office installation.
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Why downloading ucaddin.dll is unsafe
A standalone DLL download may have the wrong Office generation, architecture, language resources, or update level. It may also be modified or bundled with unwanted software. Copying it into System32 does not repair the Office registration, dependencies, Outlook add-in configuration, or Skype client that owns the file.
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Microsoft does not provide a universal standalone ucaddin.dll download for this repair. Use Office Repair, the Microsoft 365 portal, or Microsoft’s official Skype for Business Basic Client page instead.
Do not run:
regsvr32 ucaddin.dll
regsvr32 works only with DLLs that expose the required COM registration entry point. Although the Skype integration is a COM add-in, Microsoft does not recommend manually registering this Office-installed file as a general repair. The wrong regsvr32 bitness can produce entry-point or architecture errors and leave the installation inconsistent. Reinstalling or repairing the parent product performs the required registration in context.
FAQ
Is ucaddin.dll part of Windows?
No. It is associated with Skype for Business and its Outlook integration, usually installed through Office, Microsoft 365 Apps, or the matching Basic client.
Should I copy it to System32?
No. Keep it in the legitimate Office installation directory. Copying it to a Windows folder does not repair the owning application.
Does this file belong to Visual C++ Redistributable?
There is no evidence here that ucaddin.dll is a Visual C++ runtime file. It is specifically connected with Skype for Business and the Skype Meeting Outlook integration.
Why does Outlook crash when I create a Skype meeting?
A damaged or mismatched Skype add-in can cause this behavior. Repair Office, check the Office architecture, update or reinstall the matching Skype client, and then re-enable the add-in.
What if the file exists but Windows still reports an error?
The file may be corrupted, incompatible with Office bitness, missing a dependency, or part of a mismatched client build. Repair the complete Office or Skype installation rather than replacing only the DLL.
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Is SFC enough?
Usually not. SFC repairs protected Windows components, while ucaddin.dll normally belongs to Office or Skype for Business. Use Office Repair first and reserve DISM and SFC for broader Windows corruption.
What information should I provide to support?
Record the exact error wording, whether it appears in Outlook or Skype for Business, the Office bitness, whether the installation is MSI or Click-to-Run, the reported file path, whether antivirus quarantined it, and what changed immediately before the failure.
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