We upgraded ServiceDesk from 9.4 Build 9422 to 10.5 Build 10501 through the 3 upgrade packages, no issues at all in that respect and after upgrade we can log in, view all tickets, create tickets.

The issue is...we can't assign them any more!  If you try it from the main screen an error pops up Failure. Request ID(s) cannot be assigned to the selected technician, now here is where it gets weird if you click the ticket and 'edit' then select the technician that way it says "SUCCESS:Request(s) updated successfully" but the ticket remains Unassigned!

So it looks as if something is broken but it doesn't seem to be a permissions issue at the same time

At the moment I've had to roll back to 9.4 but created a copy of the 10501 to experiment with if any ideas :)