mIRC uses # as shorthand for $chan. It will try to evaluate to a channel. Although I don't know much about what should or shouldn't be allowed for channel names, I really don't think # should be a valid channel on any IRCD. You *might* be able to try something like ##, but I have no idea if that will work. You might also try using the $chr() equivalent of # and see if that works. Maybe someone else has some suggestions.

Invision Support
#Invision on irc.irchighway.net