Hi CuttingEdge, welcome to MobileRead.
Sounds like I need some validation in there - my guess is some of that Magick library doesn't like what it is being passed
To fix the issue, take a look in the plugins configuration folder for "Generate Cover.json". You can get to this folder from Preferences->Miscellaneous->Open calibre configuration directory and go to the plugins subfolder.
Now you can either just delete the Generate Cover.json file, or more usefully you might want to just manually edit it. You will see a number of entries of "margins" - whichever one you changed too large just set if back to a small number like 0. Remember it will be in multiple places in the file depending on whether you saved the settings or not.
Good luck and if you still have issues let me know.