Ive discovered that the problem only shows up when the entity behind the glass has a shader on it.
Maybe I did something wrong with my glass shader? I think it has something to do with the "cameraSpaceReflectionVector". With this I get the nice metal shining effect that is related to the camera view.
Another strange thing is, the problem only shows up when the camera is near a corner or near level blocks, Ive putted the door in the middle of a window and then it works fine. It seems that level blocks are related to this problem.

any ideas?

cheers

Zwecklos