Thanks for your bug report. I was able to reproduce this issue. This has been fixed for the next version.
If you have the "Connect on startup" option enabled, this will unfortunately cause mIRC to crash on startup, since the issue happens when the Options dialog is opened.
The only way to get around it would be to manually edit/delete the perform.ini file found in the mIRC folder. If you installed mIRC as a non-portable application, this can be found in the C:\Users\YourName\AppData\Roaming\mIRC folder.