Thank’s a lot for your help!
I tried everything from disabling all lights, deleting the lights (in a copy of the project of course) and building every level seperately, but nothing changed the fact that there is this weird light.
But the good thing is that I found an unusual fix for this unusual bug. I just put a hollow BSP at (0,0,0) and the light is concealed in it. To my luck, I don’t need to be able to enter this room, so the BSP is not interfering with gameplay.
Thank’s, and keep up the good work!