/socklisten command doesn't start successfully if there are no other sockets listening on that port, but there are sockets attempting to connect or old sockets in time_wait status (meaning they are closing the client connection)
I'm not sure about what you meant to say, but I can confirm that under some circumstances, mIRC used to report a /socklisten port error whereas all sockets were closed; I'm expecting /socklisten to work again without waiting a little bit.

Btw, there's a real bug in /socklisten, it fails to report a bind() error if you listen to the same port:
//socklisten test 8000 | socklisten test1 8000

Looking for a good help channel about mIRC? Check #mircscripting @ irc.swiftirc.net