After solving the traffic jam, I noticed that there was something wrong with the bus stop that is heading towards the outside of the town.
No bus stopped there, they turned back at the next street crossing instead and serviced the bus stop on the other side of the road. After that, they turned again (directly after the bus stop, but this may be different for my game now, as I transformed the road to 4 lanes and it goes bigger now directly after the bus stops) and headed to their original direction.
I solved this with replacing the bus stop. Maybe this is completely unrelated to the original misbehavious … but it is incorrect as well! 😀
……. naaah …. just noticing another weird behaviour: cars are turning directly below the bus stops now (maybe this is unrelated as well, but maybe I found your hidden feature, the famous “Bermuda Triangle” ?? 😀 😀 )