This is a bug I get intermittently, and it seems to happen only the first time I run mIRC after booting.
With utf & sending of utf enabled (I assume), it seems to happen. My reasoning for this is the way to get mIRC to start then is to go into the mIRC.ini and erase the lines utf=1 and mbed=1
After that, I can run mIRC, and I can even close mIRC and change the .ini back to original, and then run it fine.
To be clear, before making the above change the behavior is that mIRC begins to run, I can see the mIRC screen (but the switchbar is not yet drawn). Then it simply terminates; there is no error message.
Strange behavior, and I will probably simply turn off UTF as I don't really need it, but figured it's best to report the issues I had there anyway. Still, I wonder what could cause this? Maybe the unicode font that I am using is trying to be loaded but has some incompatibility with mIRC?