When I build my level, (with only 985 portals), I get a 'Critical Error E11c Map Size - region with too many portals'.
When I look this up in the manual it says that it has something to do with the amount of RAM.
My computer has 2 GB of RAM, I think that is enough for at least 25 times of that amount of portals.
In another level I build are over 12.000 portals and it build without any problem.
Can you explain to me what's going wrong here?

A portal is a connection between two regions of the BSP tree, and has nothing to do with your RAM.

Somewhere in your level you have a too-complex construction that can not be compiled into the BSP tree. Try to find it, and either simplify it or use detail blocks.

Or compile as 'easy map', this doesn't use the BSP algo.

Thanks, I have changed some blocks into models, now the problem has gone.

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?

Please, answer my questions;
Can models also cause this error?
Can map entities also cause this error?
Can scaled down textures also cause this error?

No.

Thanks for your extensive answer.