You have a version of mIRC later than 6.0? Preferrably the latest version (6.16 atm)... mIRC prior to 6.0 have no multi server.
Try /server -m irc.network.com (well, an actual existing server of the network you want to connect to) instead of just /server -m networkname. Maybe your mIRC doesn't have a lmist of servers for that network?
Do you have a clean mIRC downloaded from www.mirc.com
or did you download it from somewhere else? Do you use full scripts or addons?
You can try downloading mIRC again and install it to another folder to see if that version has the same problem. If not, then it's a problem with your settings or with any scripts you installed.
If that doesn't help, well, then you have something strange going on...