I have a #Doctor Who with a strange problem, when in multiball mode the balls some times stay in the Tardis. During normal play the Tardis shoots out the ball each time. When stuck sometimes it shoots the ball after a while, sometimes the DMD displays "BALL SEARCH 1", then you hear all solenoids trying to release balls, but the Tardis doesn't shoot out the ball. Is there any connection between the Tardis and the Mini Playfield?

Thanks for this advice. I cleaned all battery leakage and popped in a non volatile RAM before trying to fix anything else. Most things where easy fixes, burned limps, lose connectors etc. but this had me stuck for a while.
On your indication I started looking again in the switch test. Reading the F***ing manual I saw I had to use a ball for the test, never your finger. Since I only had experience with EM machines I didn't know this does matter.
Long story short, the ball fell to deep into the Opto Popper and it didn't get registered while it was laying in the popper. I lifted the popper a bit and all seems ok for now.