Hi @Corona_GK, thanks for your input.
As I said before, I've already enabled maps for both Realistic and Shaded, so trying that again didn't change anything. BUT changing the resolution for Baked Procedural Maps, had never crossed my mind, since this isn't procedural maps. However it actually did fix the problem - but only after saving the max-file, and then reopen it.
So for everyone with this problem: There's no immediate change in the viewports when changing the procedural resolution. You need to reload either the bitmaps or the max-file.
Thanks :)