Indeed, this is not an mIRC related problem due to the fact mIRCstats is another program in itself.
The only thing I can think of is that if the log you're trying to process is particularly big your computer might not be able to handle it - try closing other applications or try with a really small log (say 30kb or something) and see if it happens with all different sizes. Might want to try reinstalling mIRCstats.
Also might try to read any help files that come with it or, as was suggested above, contact the author.
