FTP reply code 451 indicates the server experienced an internal error that prevented it from completing the file operation. This is the FTP equivalent of an 'internal server error.' The error is on the server's side — disk I/O failures, filesystem corruption, out-of-memory conditions, or crashes in server-side processing hooks can all trigger 451. The client should treat this as a temporary failure and retry. If 451 persists, there is a systematic problem with the server that requires administrator intervention.
The server encountered a read or write error on the disk while processing the file. This can be caused by bad sectors, failing drives, or filesystem corruption. The error may be intermittent.
Some FTP servers run hooks or scripts during file operations (antivirus scanning on upload, content filtering, quota checks). If the hook crashes or returns an error, the server aborts the operation with 451.
The server process ran out of memory or hit an operating system resource limit while processing the command. This is temporary and may resolve once other operations complete and free resources.
Wait a few minutes and retry. If the error was a transient disk or memory issue, it may resolve on its own.
If the error occurs on a specific file, try accessing other files. If all operations return 451, the server has a systemic problem.
If 451 persists after multiple retries, the server is experiencing hardware or configuration issues. Contact the administrator with the exact error message and timestamp.
The file operation was not completed because the file is temporarily unavailable.
The server does not have enough disk space to complete the file operation.
The file or directory operation failed. The path does not exist, or access is denied.