See this page on the Kindle Collections plugin thread for suggestions from Kovid:
https://www.mobileread.com/forums/sho...118635&page=14
It sounds like you have either a corrupted install or a corrupted plugins directory. Try making sure there is no Generate Cover.* files in the Calibre "plugins" subfolder (Preferences->Miscellaneous->Open calibre configuration directory) and try it again. If that doesn't work, follow all of Kovid's instructions in the above thread.