I don't know what the problem is.
Try renaming the (hidden) folder .aMule in your home directory to some other name. Then run aMule again. It will create a new .aMule folder from scratch. Hopefully that problem will go away. If it does, you can remove the old, renamed one.
To rename the .aMule folder, open Applications -> Utilities -> Terminal and type:
mv .aMule saved.aMule
at the prompt.