Only the screed is broken or worse?

The house is from the 1990s, but has been unoccupied for 2 years. D

But it was hopefully heated in the meantime? If not, the water in the underfloor heating may have frozen and expanded - I'll leave the rest to your imagination... I would insist on seeing the electricity or gas bills for those two years.

In 1990, there was no underfloor heating.

?? 1990er. Not 1990. Even 1999 would still be "1990s". And underfloor heating was invented back then by the old Romans...

In 1990 there were already many houses with underfloor heating, so I would not categorically exclude it. Basically, no remote analysis can be done here, this is a task for a specialist on site.

It could be warping. I googled it once and the concave warping (sloping toward the walls) matches the cracks in the wall silicone joint.

The house has no underfloor heating and I assume it was not heated in the 2 years. It was not heated during the viewing, but *** cold

What does it cost per square meter to have the screed removed by the tiler or screed specialist?

In these boom times, no craftsman will probably be eager to take this on. Expect rejection offers; better yet: borrow a hammer drill, find a helper or possibly a janitorial service, all-rounders in the little local newspaper, and have a construction waste container brought in. Two to four days should be sufficient.

An expert before(!) purchase could also tell you whether your diagnosis is correct or whether the screed could be repaired.