The reader was
https://www.mobileread.com/forums/sho...d.php?t=198742 (librerator)
I didn't uninstall it, neither any of its prerequisites (lpad etc).
I've cleared the books on the device, yes, and removed the one I first had the issue with (it was a mobi). Like i said it froze with any action, sometimes right after the reset on the welcome screen, sometimes registering it with amazon (after reset).
Does the JB persist thru FW updates/factory reset?