Depending on your settings and the (more importantly) the access your user has to the ftp, you may be trying to rename the file after the upload OR query the status (date, name, size, etc) of the file which might not be allowed on the server.
We have a couple thousand ftp users connecting to us, from external locations, and every once in a while we get users who say they are receiving errors. The users are always able to upload files but they receive an error after the fact. Almost always it is an issue of permissions. We do not allow the users to rename or change files and their ftp software is trying, by default, to upload then rename or upload then change the date or modify the file in some sort of way.
if it is FTP make sure it is in passive mode. I am pretty sure the ftp backend, in VC, is based on filezilla software, Support should chime in as it may make a difference.
I would suggest using a 3rd part app, like filezilla or winscp, to test manually and report back your results. If all methods produce a similar result, reach out to the owner of the ftp and see if there are logs. Those would be the best way to track the issue down