Strange wall and shadows

Hey guys,
I set up an office, mostly in a hollow box brush. But when i build the lightning, a strange wall appears.

http://puu.sh/tfBaq/e007ed264f.jpg

Now, when i move the box just a little bit, it disappears:

http://puu.sh/tfBbk/2d01829ac8.jpg

But after clicking build, its here again…

And at the same time, i got strange “shadows” across the room, without anything casting it…

http://puu.sh/tfBd2/a79b2d0af7.jpg

If you need any more infos, feel free to ask.

Thanks in advance,