Originally Posted by DoctorOhh
You might want to try backing up calibre's configuration folder (unsure where it is located on Win 8. Once it is backed up delete or rename the original folder. Then start calibre it will create a fresh configuration folder on start up. If the problem is in this folder you should be back up and running. Albiet you will have to recreate your settings and add in your plugins again.
That was a GREAT idea! But, it actually made it worse; I tried one book convert and it crashed. A least my original config let me do one at a time, but not two or more.
I then ran the 32bit version and it converted that same book ok.