If you want to try a clean install, you can remove mirc.ini (you can move it somewhere like your desktop so that you can replace it later if you don't want to lose your settings).
As far as your situation, it looks like you connect just fine. Although this happens on multiple networks, one thing I notice here that could potentially be the same on those other networks is the VERSION check by the network. Many networks run that or similar things to look for bots. You might want to check and make sure that you're not blocking CTCP or aren't ignoring the bots that are running those commands. The networks could choose to kick you if you don't reply.
You could also try installing a different version of mIRC and see if that helps. For example, 6.35.
It may also help if you type /debug @debug before connecting and then paste the results of the @debug window here. It *might* provide some additional detail.