Does it always work properly in single player?
I had a similar issue, turns out one of the diverter arm in the subway was off a bit, wasn't sending the ball to the right spot.
LTG helped me figure it out.
Otherwise, I'm guessing it's most likely on opto on one of the subways exit VUKs as Thor said.
Welcome to the STTNG troubleshooting club, it can be a headache!
Off topic- I've been to Goteborg, what a BEAUTIFUL city!!!