Just a final update on this, it turned out to be a faulty M5230L IC.
After I replaced it the outputs on CN1 and CN2 are showing correct voltages now (+15 and -15V).
A big thanks to everybody who contributed to this topic 
Those M5230L IC’s are hard to find and I wasn’t able to find a substitute, so I guess I got lucky and power supply board is healthy again (I also replaced the rectifiers)
The unfortunate part, my issue in this topic is still present. I worked on the power supply board first just to eliminate that being the culprit.