It seems, that the guys at Amazon have modified their webservice interface. That’s why CoverGrabber doesn’t find any cover anymore.
The new version 0.9 fixes this problem. Please update with the PluginManager of JMP3Renamer.
!! Note !!
After the update, you also have to change the URL CoverGrabber uses. Open the settings and change
PlugIns -> CoverGrabber -> URL to