Most likely, it happens when some of the mp3 files have high-ascii characters in their names that aren't in the default system language. Accents or Nordic characters can cause this behavior.

I am aware of this problem, and will fix it when I get a moment.

This can also be fixed by changing "Language for non-Unicode programs" option in Control Panel/Regional and Language Settings to corresponding language (but I don't recommend making this change unless you know what you're doing).

Andrei