New tests: I replaced the room product I bought with Posers box primitives.

Vestman:
First render:
IDL Bounce = 3
Samples = 200
IC = 50

Second render
IDL Bounce = 10
Samples = 200
IC = 50

I think the difference is that everything gets brighter so the artifacts are not that obvious anymore. But thank you very much for the idea.

Anthnasius:
Yes, that's what I hope to get out of Poser. But I can't try because I like to get this room right first.