Yellow stains on a white wall, should they be covered with white paint?

Hello Cutepie !!!
Well, I can't really judge what kind of stains these exactly are, maybe you could remove them by scraping, but whether that still makes sense once everything is finished you have to decide.
You could isolate the remaining stains, one possibility would be to spray them with stain blocker (isolate) or the second option is to scrape off the plaster and replaster.
Regards.

Hello,
***** What are these amateur handyman tips supposed to be? Knock off the plaster again and spray the walls with a spray bottle. The stains could of course be from nicotine. It would be enough to simply paint over with nicotine barrier. I think since the spots only appear where the beams are, probably some oily glazes, wood preservatives, or stains are coming through there. Get some quick-renovation paint from a paint specialist dealer, that should solve the problem.

Regards Thommi

Hello Tommy!!
Hobby DIY tips??? well, you are something *gg*
But of course I find your version best and luckily you probably know the exact cause of the problem since you were likely on site. Unfortunately, my imagination does not go that far but I can only learn more...
Best regards

Thanks first of all for all your tips. I couldn't respond earlier because I was quite busy with the renovation and work. We have now applied [Sperrgrund] and painted over it again. The yellow stains have not reappeared so far (painting was done 2-3 days ago) and I hope it stays that way.

Thanks again, I was really panicking about having to get used to these stains, especially since our previous tenant apparently gave up the fight against them and simply painted the walls yellow.

Regards cutepie.