Thanks fo your hints – I can`t see any arrows next to effects, where should I look for those?
Here`s how i ‘bypassed’ (cause definately not solved) my problem. I Mask clips, Disable all Track/Clip effects – then I have a clean mask. I render this to a file and do Effects in another session.
Apparently these lines/shadows ONLY appear when using any sort of effects on the clip/track that is beeing masked. That`s the most stupid bug i can think of, I don`t know how they could let it happen.