The actual entire error I was getting: ERROR ITMS-9000: "File given does not match type: JPEG Image" at Book (MZItmspBookPackage)
I tried several things, but ultimately the fix seemed to come after I changed the cover to a .png file (instead of a .jpg).
Just in case anyone hasn't tried that fix yet. Let others know if that also resolves the problem for you.
Regards,
Brian Schwartz
www.KindleExpert.com