Login as admin and try to perform an "On demand ovewrite" from Tools menu at the Security section.
And if this not solve your issue do a software upgrade or reinstall by AltBoot.
I have a Xerox WC 7845 that gets stuck preprocessing every job. It will clear the top priority job after a reboot but then everything else gets stuck preprocessing. Have tried printing from Server and PC. Tried PCL6, PS, and Global Print Drivers.