On some PC's we've started getting the following error.

Internal error 0XFFFFFD5F occurred. Try again. If it reoccurs... restart.
If it persists, contact your network administrator.

It may appear only once on one PC, or repeatedly on others. Has to be
rebooted before you can login.
Its happening on clients 4.91, 4.91 SP1 and 4.91 SP2.

Anyone else had this problem?