Illumination Occluding Crash

I googled it and I'm not the first one with this problem but I can't figure out solution. I have boxy brush, and when I want to build geometry, first progress bar pass finishes and in the middle of the second pass ("Illumination Occluding") editor just vanishes away. I move brush somewhere else and build works. No matter if it is just geometry build or full rebuild, always crashes on that second pass.

I need the brush on THAT place and I'm a little bit desperade. I ran into this problem before, but I just remade some brushes in other way. But here I have not so many options. And... I don't want to "hack" brushes (pure boxes in this case!) always when this happens.

Somewhere I read, that it can be light in the geometry... not my case now.

Thanks for any hint

Well it's more serious than I was afrait... from the morning I simply could not add any subtraction brush so I ended up with the solution I read somewhere - delete all lights (except few I was certain about). That helped but... is there any normal solution how to track down the problem? Maybe some light really get into geometry somewhere on the other end of the map time before (when some inner wall was build) and it did not showed up til yesterday.

Eh... I'm not sure of anything sometimes with the UEd.

I was not sure it was a ligth, but it turned out so appearently. I will stop to use "temporary" lights because they like to get into later added geometry. Bad bad lights.

But still... really stupid way how Editor telsl it to the user. ;-)