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Riot does not currently publish a clear definition or a guaranteed fix for a standalone League of Legends “0U” error. First verify whether the code is actually 0U, OU, or another message; then check for a service incident and work through low-risk client, network, and security-software checks. Do not change BIOS or Vanguard settings unless the actual error begins with VAN or explicitly names those settings.
What does error 0U mean?
Riot’s indexed League support materials do not define “0U” as a standalone error code. The short text may be a client login, patching, or connection symptom, a temporary issue, or a misread character. User-uploaded videos describe similar-looking errors as client or login problems, but they do not establish one official cause or a dependable fix. Riot’s League support hub lists separate troubleshooting categories, including launch, patching, connection, and Vanguard issues.
Before troubleshooting, take a screenshot and inspect the full message. A zero and the letter O can look alike, and the code may be part of a longer error. Online guides may describe an older client or a particular patch incident, so treat a fix as relevant only if it matches what your client actually shows.
Check whether Riot services or a patch are affected
Start with Riot’s official service-status information through the League support hub, and look for announcements for your region. Note the region and the time the error appeared. A regional login or authentication incident can affect sign-in even when the game itself is not broadly down; a patch deployment can also cause update failures without indicating damaged files on your PC.
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- If Riot reports an incident or other players in your region are affected: wait for Riot’s resolution and try again later. Reinstalling during an active outage will not repair the service.
- If the error appears during an update: treat it as a possible patcher or file-access issue and proceed to client repair.
- If status information shows no incident: continue with local checks. An operational status does not by itself prove that your PC is the cause.
Fully restart the Riot Client and Windows
- Close League of Legends and exit the Riot Client from the notification area/system tray, rather than just closing its window.
- If the client remains stuck, open Task Manager and end only clearly identified Riot or League processes that are still running.
- Restart Windows, open Riot Client normally, and try launching League once.
Restarting is a low-risk way to clear a stuck client process, but it cannot resolve a service outage or a persistent network or security-software block. Riot’s Vanguard troubleshooting also uses restarting the client and computer as an initial check for several Vanguard error categories; that does not mean 0U is a Vanguard code. Riot’s Vanguard error-code guidance
Repair League of Legends in Riot Client
- Open Riot Client and select the League of Legends game page.
- Open the client’s Settings and look for Repair, Full Repair, or a comparable file-verification option. Labels and availability can vary by client version.
- Start the repair and let it finish without closing the client. Then restart the client and test the game.
If no repair option is visible, do not download an unofficial repair utility. Use the current instructions in Riot’s support hub for the client version you have.
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Check VPNs, security software, and overlays
These are possible blockers, not confirmed causes of 0U. Test one change at a time so you can identify whether it matters:
- Disconnect a VPN or proxy, then retry. If other network symptoms are present, restart your router as a separate test.
- Close overlays and hardware or monitoring utilities, such as GPU overlays, FPS monitors, RGB-control software, streaming overlays, and aggressive network filters.
- Check whether Windows Firewall or third-party security software is blocking Riot Client or League. Update antivirus definitions before considering an exception.
- If it is safe and allowed by your security product, briefly pause third-party antivirus protection to test, then restore it immediately. If the test identifies a block, use Riot’s current support instructions to create a narrowly scoped exception rather than leaving protection disabled.
Riot has documented antivirus interference with particular League or Vanguard components after specific updates, but those reports are patch-specific; they do not establish a universal 0U fix or a permanent executable path. Riot’s patch-specific antivirus guidance
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Flush Windows DNS cache if connection symptoms point to a local network issue
On Windows, open Command Prompt as an administrator and run:
ipconfig /flushdnsWindows should report that it successfully flushed the DNS Resolver Cache. This clears the local DNS cache; it is a general network diagnostic, not an official 0U-specific repair. It does not fix an outage or corrupted game files. Do not start with router port forwarding or changing DNS providers for an undefined code.
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Use a clean boot or reinstall only after simpler checks
If the client still fails and you suspect a background program, use Riot’s clean-boot guidance from the League support hub to test whether another startup service is interfering. A clean boot temporarily changes which Windows services and startup apps run; follow Riot’s steps to restore the normal startup configuration after testing.
Reinstalling League is a later option if repair fails and the problem began after a failed or incomplete patch. It can replace damaged files, but it will not fix a regional service incident, account authentication issue, or background network block. Do not repeatedly reinstall if the same error returns. Remove Vanguard only when a Vanguard-specific error or Riot Support instruction calls for it, and avoid registry cleaners, unofficial uninstall scripts, and replacement DLL downloads.
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If the message is actually a VAN error
Vanguard errors generally use the VAN prefix and have their own instructions. Do not apply TPM, UEFI, Secure Boot, virtualization-based security, or Exploit Protection changes to a screen showing only 0U. For example, Riot documents VAN 9002 as an Exploit Protection issue with its own procedure: Riot’s VAN 9002 instructions. Use the article matching the exact VAN code shown. Riot’s Vanguard FAQ explains what Vanguard is, but does not identify 0U as a Vanguard error.
Send Riot Support the details and client logs
If the error persists after the checks that match your symptoms, submit a ticket through Riot Support. For unknown patching errors, Riot specifically asks players to attach League Client logs. Its patcher troubleshooting article lists the default Windows installation folder as C:Riot GamesLeague of Legends; the relevant logs are in the game’s Logs folder, including LeagueClient Logs. On macOS, open the League application contents and go to Contents/LoL/Logs. Follow Riot’s instructions to compress and attach the logs.
Include these details so Support can distinguish a client, patch, account, or connection problem:
Quick Recap
- A screenshot and the complete error text; specify whether it reads 0U, OU, VAN, or something else.
- Your region, operating system and version, and Riot Client or League version if shown.
- When it started and whether it followed a patch, driver update, antivirus change, or Windows update.
- Whether other Riot games or accounts are affected.
- What you tried and what happened after each step, plus the compressed logs.
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