I know this is an old question, but I run in the same problem and wasted too much time on this, so maybe it will help someone else in the future.
I didn't understand exactly the reason for this, but it seems the problem appears when you play with time values on an arch that ends to a transition (I was updating an integer value to the current time, using IntInf.toInt(time())). Now, if I move the code on the outgoing arch of that transition (that is: the one that ends in a place) there is no error.