The issue seems to be caused by a specific chain of events: The AGV is travelling to a destination CP in it's "idle-loop". It arrives at a traffic control zone and is preempted and made to stop at the same CP is was travelling to previously. If the AGV is now resumed, the original travel task finishes instantly. This also triggers the On Arrival event that a third token somewhere else was waiting for to start giving out a Task Sequence to the AGV. This and the new "idle-loop" task seems to 'collide' in some way and cause the error. I was able to replicate this in a simple example model. I have attached it if you interested in checking it out.
Making sure that the "idle-loop" does not include CPs that represent an entry to a traffic control zone seems to be a possible workaround. Specifically I replaced CP5 with CP3 in the first travel activity of the OHT2 idle loop.
