It looks like maybe a comflict in pb, the ui tries loading which should set an active work coordinate system. This is how the button synchronization and exclusivity functions.
It may be your pendant code. This is the first error of that nature ive had reported in 8 years and there are thousands of pendant users out there.
I would try moving any items that need to synchronize with the gui into the postgui halfile and see if functionality still works and the error goes away.
