Blind stab in the blue: Have you used ZORK to modify the system-font?
Then there might be a faulty prsp.sh file in /database/system/PRSPlus/
Remove it and try to restart the reader (not connected to USB and with PRS+ installed)
You can also try and download a special prsp.sh
and place it into /database/system/PRSPlus/
again restart with reader not connected to USB.
In theory this startup-script should delete all PRS+ settings.
If successful don't forget to delete/rename this file afterwards.