The error came back after a while. After deleting some blocks I found that it was caused by a block with a round hole in it, made by CSG Substract.
Verry nasty way to create a hole.

Edit: the error is back again, now I don't know what to do about this anymore. I got rid of the block with the hole, changed a lot of blocks into a map entity and changed a lot of blocks into a model.
When I remove the map entity and the model from the level this error still comes up. The level is now verry empty, only some models are on it.
Can models or/and map entities cause this error or is there still something else I have to look for?
Or can scaled down textures cause this problem?