Maybe you had a modified QINTER subsystem which has been replaced when you’ve upgraded the OS?
I would suggest checking the routing entries in your QINTER and “work” subsystems. Take a look at the workstation entries, maybe they have changed since the upgrade. It appears that you have a conflict between the definitions of the 2 subsystems and the 2 are “fighting” over which subsystem the job gets allocated to. So, when the subsystems are busy allocating jobs in your rush hours, you get the allocation problem, but when they’re less busy there’s no problem.