So on Windows, if the file is not completed and the transfer stops,
then the file is removed from the server.
Its likely therefore that the issue is the ability to complete a
transfer. I have been going around with Apple on this "bug" and they
tell me it may be fixed in 5.x, and can I try using that version of
Beta IOS... unfortunately nobody actually having the problem is on 5.x
and they won't let me give it to anyone.
I suggest trying:
An iPhone reboot.
A server reboot. (less likely solves it)
Switching Active/Passive mode