I haven't found a topic with a solution for this problem in the forum. If I missed it, sorry, and I'd be happily redirected there. I found the similar thread https://pinside.com/pinball/forum/topic/far-out-scoring-reels-advancing-in-tandem-bonus-constantly-lit , that helped, but was not enough for me to solve the problem.

You can find the full background story to the pin etc here: https://pinside.com/pinball/forum/topic/surf-champ-start-up-issues-ball-eject-and-reset-banks
In short:
Game and location
Gottlieb Surf Champ, 1976
Bern, Switzerland (230 V)
I have a multi-meter and soldering tools and know how to use them. I have the schematics and game operation instructions.

The issue:
Every time 500 points are added to the 2nd player, 4000 are added at the same time. This only happens for the 2nd player, all the other points are added correctly.
As you can see in the following videos, the L relay (1000 points relay) fires at the same time as does the M relay (100 points relay).

https://www.dropbox.com/s/5lk48y5wru05i4l/20230317_162328.mp4?dl=0
https://www.dropbox.com/s/ndk88l4bxp319w8/20230317_162457.mp4?dl=0

The 9th position switch on the "hundreds" reel seem to be correctly open when they should, but I might be wrong. Here's some close up pictures:
Position 8
Position 9
Position 0
Position 1

Any help would be highly appreciated.

20230318_112536 (resized).jpg

20230318_112545 (resized).jpg

20230318_112556 (resized).jpg

20230318_112604 (resized).jpg

pinballdaveh you are amazing! That was indeed the problem. I opened the switch and now it's working like a charm.
Once the problem is clear, it feels odd that one has not seen it before...
Thanks for the super fast help!