òk it seems that was the problem. Or, at least. I removed all the lights and put all new ones in and everything seemed fine. Thanx for the tip.
But now, when i'm rendering, it gives black beams where my lights are, whether i'm reversing the faces or not... and the sun through my windows doesn't leave the bright area's on the floor I would like. it doesn't leave any marks on the floor at all actually.
Ah well, i guess it was stupid to try this out while having a deadline.