We have this issue as well and have never traced it to a root cause.  We lose between 1-2% of the items attached through DocMan to the ftp server.  There is no explanation for it really.  We tend to blame it on the user doing it on a wireless connection and the connection dropping or losing connectivity at the point of saving.

The only way to fix it is to find the original and attach it again.  We’ve not found a way to recover the files from the ftp server once you receive this message.