Yeah, those two bugs “thralls must be placed on walkable ground” and “sorcery benches dont work (too far away)” seem to be related as they appear in same areas.

And someone suggested it has something to do with altitude rather than position on the map. So far it seems it could be the case. I tried placing stuff on one of the icy ridges, didnt work. But it did work on the foot of said ridge.

At any rate, the problems appear in the high areas around (and possibly in) the volcano