black spots

Here are a few more pictures, maybe you can see more there.


How do I best approach this?

You have to google it. For example, mold lab testing.
There are quite a few companies in the next larger cities; they also have a homepage describing what kind of examination is necessary, for example room conditions, substrate, photo with scale, etc.
In the second picture, it looks like small ants ;)

I would say these are some kind of little creatures. I mean, in the pictures you can see "little legs" on some of the objects and partly also something like a segmented body.

I also see little legs.

Google "Gewitterwürmchen" or "Gewittertierchen" For me, it looks like this...

I am missing the tape measure in the photos to judge the size...

Thank you for the tip Gewitterwürmchen, I found another post because of it and it fits our matter like a glove, so they seem to be such creatures.