Masonry - Closing/filling the wall behind the underfloor heating manifold

Finish plaster? Gypsum plaster (from Rimano)? Or?

Theoretically, you can apply any plaster there. But since you will probably apply the same as is already on the wall, it will probably be a gypsum plaster. Which gypsum plaster doesn’t matter. If you have too much money, a plaster based on gypsum filler also works. In the end, it will be the same thing your plasterer did. The primer couldn’t have been so wet that the plaster stays damp for so long. If it was Styrofoam, maybe he applied some kind of scratch filler with an adhesive thinly. What I suggested, for example, is nothing else except that it is not Styrofoam, but a rigid foam that already has the primer on it. The main moisture is in the gypsum plaster and if there was no other cause, the drying just takes time. Under bad conditions, you wait for weeks (even if the weather doesn’t suggest it right now).

Logically, the same material that was used for the rest of the wall. At least the type of plaster, e.g. gypsum plaster. Otherwise, the surface will be completely different, which you probably don’t want; as a layman, you won’t be able to make it "disappear" anyway.

What do you mean by ready-mixed plaster? Something from a bucket? No. Sack, mix it yourself.

I still find the question interesting why it stayed wet. That’s actually not normal. How thick was the plaster?