How interesting! For some reason windows FTP sockets are reporting errors. Since I've never seen such a thing ever before I don't have an immediate answer.
This does not come from Opus but from Windows Socket functions.
"listen() - WSAENOBUFS: No buffer space available "
It looks like some strange issue with an AntiSpyware program where the data sockets are being blocked from accessing the connection. Check that Opus is allowed access through your firewalls or better still disable your AntiSpyware etc temporarily and retry.
If you can't work it out, please talk to me privately (greg at gpsoft dot com dot au) on this with some more details and I'll run a debug session and see what I can find.
I don't know for sure but it could also be that "feature" added to XP SP2 that drops outgoing connections if there are too many pending connections. That shouldn't be a problem unless the machine is making lots of other connections at the same time, but if that is the case then it could be relevant.
I can not disable our AntiVirus software - McAfee VirusScan Enterprise 8.0.0 (since 11th May 2007).
It would be always important if the user would see run time errors like this in an error window. If I would have seen this error message I wouldn't use the forum title 'ftp falls back to c:'.