Apparently, an exploit was discovered earlier today in the current version of mIRC. The exploit causes mIRC to quit just by sending it a DCC request. As of right now, the only way to fix the problem is to disable DCC completly. The versions affected are 6.0-6.11 (current).
The author has been contacted, but no official word has been released by him about the exploit.
To disable DCC, just type the following command, on a new line by itself in any chat window, beginning with the slash character:
/ignore -wd *
Keep in mind this is just a temporary fix.
News source: More info on exploit on irchelp.org