When a file is deleted to the recycle bin on a remote server, WinSCP apparently tries to first move the file to /tmp/filename-datestamp.ext. On servers that do not have a writeable /tmp (Windows being one example), this fails.
This behavior seems to occur even if none of the "Recycle bin" checkboxes are selected.
I'm not sure what the intended behavior is, but can there please be an option to completely disable the "Recycle Bin"?
WinSCP 4.1.8 (b415) on WinXP
A side note: On Windows servers, the recycle bin is user-specific and not at an easily-guessable path, since it requires a call to the well-known-path API. Expecting users to know the recycle bin location and their user SID so they can provide the path to the remote recycle bin (assuming it's even in the default location) seems overly optimistic. If you want to have such a feature, I'd suggest WinSCP attempt to discover this path somehow for the user through well-known-paths. I'm not sure how scp or sftp (much less ftp) would be able to execute arbitrary code as the user on the remote server though. Maybe upload an executable and try an exec request to run it and get the output?