From littlecramp's response,
here, the problem is with mIRC's DDE Server setting, which accepts links from other Windows applications. In mIRC, go to:
File -> Options -> General -> Servers, check Enable DDE Server
If Enable DDE Server is already checked, just uncheck it and recheck it, then restart mIRC.
I forgot to look in mirc.ini to see if the DDE initialization values were incorrect there before I made the correction. The best (and easiest) way to correct the problem is probably through the Tools menu; though I'm curious to know what the mirc.ini DDE and DDEServer values were before I corrected them.
