FileZilla’s “Could not start transfer” line is a queue result, not a diagnosis. The useful error is normally in the log immediately before it. Read that line first, then check the local file or destination, the remote directory and account permissions, and the separate FTP data connection.
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Most cases can be isolated without changing random settings: establish whether the failure is local, remote, or network-related; use passive mode as the starting point; run FileZilla’s Network Configuration Wizard; and involve the server administrator when the account, quota, or server-side port configuration is the blocker.
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Run this troubleshooting sequence
- Capture the complete log. Expand the failed queue item and copy the lines before “Could not start transfer.” Record whether it is an upload or download, the local and remote paths, and the first explicit error.
- Verify the local side. For an upload, confirm the source still exists and can be read. For a download, confirm the destination folder exists and is writable. Check for a file lock or a protected directory.
- Verify the remote side. Make sure the account is in the intended directory and has read permission for a download or write permission for an upload. Try a directory that the account is known to use.
- Use passive mode first. FTP has a control connection and a separate data connection. Passive mode is the normal choice for a client behind a router, NAT, or firewall.
- Run the Network Configuration Wizard. It helps identify whether FileZilla can make the outgoing connections required by your network. Review firewall and antivirus logs at the same time.
- Separate client, network, and server responsibility. A successful login does not prove that directory listing or file transfer is possible. Determine which layer fails before changing more settings.
- Keep encryption compatible. Prefer FTP over TLS when the server supports it. Use plain FTP only when the server operator confirms that compatibility requires it.
- Investigate timeouts on long transfers. If small files work but a large transfer fails near completion, look for routers or firewalls dropping an idle control connection.
What “Could not start transfer” actually tells you
The phrase is a generic failed-transfer status. It does not identify a permission problem, a firewall problem, a missing file, or a FileZilla defect by itself. FileZilla’s translation catalog uses the same status text for a failed queue item, and support examples show it appearing alongside a more specific local file-open or read error.
Use the queue message as a pointer to the log, not as a reason to reset every setting. The first explicit error usually has more diagnostic value than the final queue summary. For example, a local read failure points to the computer running FileZilla; a permission response points to the FTP account or server; and a data-connection timeout points to the network path or server port configuration.
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Read and preserve the full log
Before retrying, expand the message pane and scroll to the beginning of the failed operation. Copy the complete block rather than only the final red line. The order of events matters: login, directory navigation, permission responses, data-channel negotiation, and the eventual queue status can distinguish a local problem from a transfer-path problem.
Write down four details:
- whether the operation is an upload or a download;
- the exact local path and remote path;
- whether directory listing succeeds;
- the first explicit error and when it appears.
Keep this record for an administrator or hosting provider if escalation is necessary. A complete log is far more useful than a screenshot containing only “Could not start transfer.”
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When an upload fails
FileZilla must open the local source before it can send anything. Confirm that the file still exists at the path shown in the queue; files moved, renamed, deleted, or replaced while a queue is waiting can produce a local-open error. Open the file’s properties outside FileZilla and verify that your account can read it.
Close applications that may hold the file exclusively. Editors, archive tools, game launchers, synchronization clients, and security scanners can temporarily lock a file. If the file is in a protected system or application directory, copy it to a user-owned folder and retry from that copy. A successful transfer from the copy isolates the problem to local access rather than FTP.
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When a download fails
A download needs a local destination that FileZilla can create or write to. Confirm that the destination folder still exists, that the drive is available, and that your account has write permission. A read-only location, a disconnected drive, or a folder protected by operating-system policy can prevent the data channel from opening even when the remote account is valid.
Choose a simple, writable test folder and download a small file there. If that works, the original destination—not the FTP server—is the next place to investigate. Do not delete or overwrite the original file until you know whether the failed item is a local access problem.
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An FTP login can succeed while the account is barred from the directory or operation you requested. Confirm that the remote pane is in the intended directory, not a similarly named folder or an account root with different restrictions.
- An upload requires write permission in the remote directory.
- A download requires read permission for the remote file and access to its directory.
- A protected directory, server-side lock, storage quota, or account policy can block a transfer even when listing is visible.
Try a directory that the account is documented or known to use. If a small file transfers there, the original path is governed by a different permission or policy. Do not attempt to bypass a protected directory; the server administrator or hosting provider controls those permissions, quotas, and account rules.
Why login can work while transfers fail
FTP uses two connections. The control connection handles login and commands. A separate data connection carries directory listings and file contents. Therefore, a successful connection to the server—and even a successful login on the usual control port—does not prove that the data connection can be established.
This distinction explains several common patterns:
- Login succeeds but the directory never lists: investigate data-channel negotiation, firewall rules, NAT, and the server’s passive-port configuration.
- Listing works but one upload or download fails: investigate the local file, destination, remote permissions, quotas, or locks.
- Small transfers work but a large one fails late: investigate idle-connection timeouts and stateful network devices.
Passive mode versus active mode
FileZilla’s official network guidance recommends passive mode for typical clients. In its wording: “In passive mode, which is recommended (see below), the client sends the PASV command to the server, and the server responds with an address.” The connection direction and the ports involved are the reason this choice matters.
| Mode | How the data connection is established | NAT and firewall implications | Server-side considerations |
|---|---|---|---|
| Passive | The client sends PASV; the server responds with an address, and the client opens the data connection. |
Usually suits clients behind NAT or a firewall because the client makes outgoing connections. The server may select a data-port range. | The server’s passive address and port range must be correctly configured and reachable. |
| Active | The server connects back to the client for the data channel. | More sensitive to client firewalls and NAT. Behind NAT, the external IP and a forwarded port range must be correct. | The server must be able to reach the client’s advertised address and port. |
Set FileZilla to passive mode in its FTP transfer settings unless the server operator specifically requires active mode. If passive mode still fails, do not assume active mode is automatically better: active mode may require firewall exceptions, a fixed external address, and port forwarding.
Use the Network Configuration Wizard and inspect security software
FileZilla’s official guidance says, “If you’re running FileZilla 3, it’s recommended you run the network configuration wizard.” Open the wizard from the Edit menu and follow its tests with the same connection settings that produced the failure.
FileZilla must be allowed to open outgoing connections. Check the operating-system firewall, endpoint security, and antivirus logs for blocked FileZilla processes or blocked FTP data ports. A security product can permit the control connection on the usual FTP port while denying the separate data connection.
Passive mode can involve server-selected ports, so a local firewall rule that permits only the control port may still be insufficient. On the server side, the passive port range and advertised address must be valid. Active mode behind NAT requires the correct external IP and a forwarded port range. These server-side values are controlled by the server operator, not by a setting in the FileZilla queue.
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Testing from another network can help separate a local firewall or router from the account and server, but do it only when appropriate for the connection and security policy. A different network is a diagnostic comparison, not a permanent workaround for an incorrectly configured server.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose FTP over TLS carefully
FTP over TLS is the preferred option when the server supports it. It protects the session while retaining FTP compatibility, but the server must offer and correctly negotiate TLS.
Best Value
| Connection choice | When to use it | Protection and trade-off |
|---|---|---|
| FTP over TLS | Use when the server supports TLS and its certificate and policy are compatible with the account. | Preferred for protecting credentials and transfer data. A TLS requirement can expose compatibility or certificate problems that plain FTP does not. |
| Plain FTP | Use only when the server operator confirms that compatibility requires it. | Credentials and data travel without encryption, so it should not be treated as a general fix for transfer errors. |
Changing encryption will not repair a missing local file, a denied remote permission, or an unreachable data port. Treat it as a compatibility decision after the log identifies a negotiation or server-policy issue.
Diagnose failures that happen near the end
During a transfer, the data channel carries the file while the control channel may sit idle. Some routers and firewalls drop connections that appear idle even though the file is still moving on the data channel. The result can be a large transfer that fails close to completion while small files succeed.
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Interpret common log patterns
| Pattern in the log | Most useful next check |
|---|---|
| A local file-open or read error appears before the queue status. | Verify that the source exists, is readable, unlocked, and outside a protected directory; for downloads, verify a writable destination. |
| Login succeeds, but listing or transfer cannot open a data connection. | Use passive mode, run the Network Configuration Wizard, and inspect firewall, antivirus, NAT, and passive-port settings. |
| The server rejects an upload or download in a specific directory. | Check the account’s write or read permission and try a permitted directory. Ask the server administrator about locks, quotas, or policy. |
| Small files succeed, but a long transfer fails near completion. | Investigate idle control-channel timeouts in routers, firewalls, and server configuration. |
| Only plain FTP works, or TLS negotiation is rejected. | Ask the server operator which encryption mode and certificate policy the account requires; do not disable protection merely to make a retry pass. |
When to contact the server administrator or hosting provider
Escalate when the log points to a protected directory, quota, account policy, server lock, passive-port range, or advertised passive address. Those settings are outside FileZilla’s control. Include the full log, the operation type, local and remote paths, the directory that worked or failed, the transfer size, and whether passive mode was enabled.
Ask the operator to verify the account’s read or write permission, available quota, server-side locks, passive data-port range, NAT or external-address configuration, and timeout policy. This gives the operator specific checks instead of a vague report that “FileZilla cannot transfer.”
Quick Recap
Prevent the error on future transfers
- Keep source files in locations your operating-system account can read and destination folders in locations it can write.
- Close applications that hold files open before queuing an upload.
- Use passive mode as the default for clients behind NAT or firewalls.
- Allow FileZilla’s outgoing connections and review security-software logs after policy changes.
- Use FTP over TLS whenever the server supports it and documents the required mode.
- For large files, monitor router and firewall idle-timeout behavior rather than assuming a late failure is a corrupt file.
- Save a representative log when contacting the server operator so the same permission or data-port issue can be identified quickly.
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