This has only recently started happening - I can't work out why.

I'm using FileZilla (latest version) to FTP files up to a Windows 2008 R2 server on IIS7.5.

Every 10th (but not always every 10th) file occasional fails to complete the upload (no error message), so tries again. On it's second attempt the file can't be uploaded and receive this error:

Response:   550 The process cannot access the file because it is being used by another process. 
Error:  Critical file transfer error

There is no pattern to this...and no explanation. The only way to get around it is to restart the FTP service on the server - it is the FTP service that is keeping a lock on the file.

A couple of things:

  • There is no file size that causes it, it happens whether the file is 1kb or 1000!
  • Looking at the "current sessions" on the server, there are 10's of sessions on there that say "Current Command" STOR - FileZilla is not currently connected. So my assumption is that the connection is failing during a transfer, but the server thinks it's still uploading so keeps it's own connection open...
  • I've disabled the firewall and anti virus, so they're not the cause
  • Same happens for Active and Passive

Any ideas?

This is essentially the same problem: Win 2008/FTP/Filezilla - Intermittent critical transfer errors (no answers)


I found the answer to this - thought I'd post this as it may help someone else diagnose the same problem

The problem was AVG. For some reason, AVG was killing the connection for some files (but not all), but killing the connection between the client and the server meant that both ends of the connection thought it was still open - so locked files.

  • instead of replacing the file, i just deleted that file from server and uploaded again, now the error is gone. Hope helps someone. – shaijut Jan 9 '16 at 8:40

The same symptom has been identified in Windows Server 2008 R2 when sharing violations occur due literal size commands, and has been published as KB2723776

File resumption commands for the FTP implementation were not too reliable in IIS 7.0 and is still not perfect in 7.5

I suspect that the hotfix mentioned above fixes more than just the specific issue with literal size commands.

  • It was a good shout! Alas it doesn't appear to have resolved the issue – Paul Dec 17 '13 at 19:44

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