No error message at all, even after clicking on the dialog? I just tested it with aMule 2.1.3 and I got a nice crash. (Seems to be a bug with aMule 2.1.3 - this doesn't happen with developmental versions)
Too bad, you should be able to solve the problem anyway:
Open the file ~/.aMule/amule.conf (~ is your home folder, e.g. /home/jossaq/) and search for the entry "UseSkinFile=1" and change the value to zero. Then it should start fine again.
Remark after reading this again: Dear wuischke, please don't think faster than you can write (who turns "just tested" into "justed" anyway?) and doublecheck your sentences for spelling mistakes.