No the map entity isn´t set to "transparent". In a testlevel I was able to make a mirror with the same script, without errors. So it has to be something in my script (maybe clip_far??? I´ll check that) or in my level, but it´s a very simple level so far...
Thanks for your replies, btw.

edit: setting the mirrors clip_far and clip_near values to the camera values doesn´t change anything.
Noticed another thing:


^^When the camera tilt is nearly 0 the mirror looks like it should. If I rise the camera and it tilt changes, it looks like in the last pic.