If you used the /debug command to show traffic and nothing shows up for private message, then it means the server is not sending anything somehow, or that you have a script conflicting, but that's unlikely.
You say it's related to your installation, if you create a new folder, put a blank mirc.ini file in it, and a copy of your mirc.exe file, and run that mirc.exe in the new folder, can you see private message?
