Remove old building floor covering. The adhesive simply won't come off.

I dealt a bit more with the topic (too late). It could be that the glue contains asbestos. I am now sending a sample to the lab and will only enter the room wearing an FFP3 mask anyway. If there really is asbestos in it, which is unfortunately likely, I will call in a specialist company. It will surely cost me 1000€ for the small room. But that's just how it is.

Dear people, I can only tell you: "Please keep the church in the village!!"
Otherwise, there might still be a posting with the suspicion that the adhesive layer contains malaria.
What do we see in the pictures from "shooty76"?
We see that the last flooring was installed with the appropriate adhesive notching on a leveling compound layer.
And this leveling compound layer is detaching from another, darker-colored leveling compound layer.
This means that the then new screed was reworked with a leveling compound, a (first) flooring was laid on this, and later dismantled.
Even back then, they didn't want to bother removing the first leveling compound layer from the screed.
So they mostly removed the adhesive ribs, but they are still partially visible.
Then a second leveling compound layer was applied on this level, which after laying the last flooring and after many years of use no longer adheres properly to the lower leveling compound layer.
This means for the upcoming flooring installation:
Grind down all leveling compound layers until the aggregate (screed) is exposed!
Vacuum the dust well, prime the entire surface and completely level again in a thickness of about 2 mm. Probably, there will also be isolated cracks in the screed here and there, which are either filled with resin or covered with a suitable glass fleece strip (the latter does not comply with technical standards but is state of the art).
Anything else considered in the process is not sensible and will not lead to success (a damage-free new installation)!!
However, one may (perhaps) recognize that there are trades in construction that require expertise if the trade is to succeed.
Expertise can never be replaced by overestimating one’s own DIY skills.
Of course, a specialist company can be called in for the dismantling. Grinding costs about €7.50/m².
But for €1,000 (unless the room is over 130 m²), any flooring company is happy to come...
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Wishing you good success KlaRa

Thank you very much for the numerous responses.

However, as the seller told me, this is the first flooring. It would then have had to last 50 years, could that be possible? It didn’t look that bad either.
I really fear, however, that the adhesive is really nasty stuff. When you scrape off the fabric, you can also see that there are grooves in the black layer above the screed. Comparable to the grooves you make in tile adhesive. I am pretty sure it is glue and then PVC was laid on top of it.

Anyway, I don’t dare to approach the black stuff with any sanding machine.

When the laboratory results come in, I will get back to you. Hopefully KlaRa is right and it is really harmless.

Has anyone ever experienced similarly nasty surprises when removing flooring?

Thank you very much for the numerous responses.

However, as the seller told me, this is the first flooring. It should have lasted 50 years then, can that be possible? It didn’t look that bad either.
I really fear, however, that the adhesive is really awful stuff. When you scrape off the fabric, you can also see that there are grooves in the black layer above the screed. Comparable to the grooves you see in tile adhesive. I am pretty sure that it is glue and that PVC was laid on top of it.

In any case, I don’t dare to touch the black stuff with any sanding machine.

When the lab results come back, I will report again. Hopefully KlaRa is right and it is really harmless.

Has anyone ever had similarly nasty surprises when removing the floor?

How did it continue!?
We are soon facing the same problem!