My guess would be bad data somewhere in the calendar. Check all of your appointments and see if you can find anything strange like weird characters or block letters.
Otherwise you might have some sort of corruption in the calendar app and you might want to try a hard reset after a backup of your data.

I'm currently seeking a new calendar widget as the one i've been using for some time has been crashing a lot. (Android Agenda Widget) Worked pretty good at first and has a screen widget screen. Now crashes on startup but I can't find anything to replace it with.