Maarten,
I suspect that PM's cache folder has become damaged. I suggest manually emptying it. To do this, quit PM, then switch to the Finder. On the Go menu choose "Go to Folder..." and enter:
~/Library/Caches/
Look for the "Photo Mechanic" directory and drag it to the Trash. Empty the Trash.
Launch PM again. If this doesn't solve your problem, please let me know.
-Kirk