I'm also getting the "Copying files to remote side failed." error on a ProFTPD Version 1.3.1 server using Explicit SSL or TLS. It appears whenever I attempt to copy a file off the server. Unencrypted FTP does not seem to have the same problem.
There is also a redrawing issue using the Login dialogues Alt shortcuts. Some controls will disappear with the exception of the underline underneath the shortcuts key. Appears to only happen once per category, but it can happen to numerous categories. Clearly happens when Alt+Tabbing, but depending on how Windows redraws the currently visible windows it might correct itself. To reproduce, just open up WinSCP and press Alt.
Both issues observed on Vista 64 SP1 using WinSCP 4.2.1 beta.