Hello everyone,
we have found and fixed the reason for the crashes!
The underlying issue was a misunderstanding of Unity's handling of materials (the thing that says how a object looks). It used to be a small issues since materials don't take much memory at all.
But our visualization modes change the materials of a lot of buildings and over long play sessions it added up and crashed some players' games if they had little RAM left.
Thanks for your help in fixing it!
Happy playing!