Update on my situation:
When I connected my tab to my pc (with the correct drivers, I've always had the correct ones) nothing happend.
Switching MTP on, did't do anything, when I went to another settings panel and came back to MTP, it was switched off again.
So I did a full wipe (cache, factory, dalvik) and reinstalled nighlty 21.
And suddenly everything was ok. I could connect with my pc again, my CPU finally went back to lowest setting (it was constantly 1000 before) and youtube videos in HD play again as the same before ( an occasional stutter instead of constant 2-3 sec stutters).
What I think happend (I am a total noob, so I have no clue, haha). When I was on 13 nightly, my MTP was switched off.
Going to 15 nightly (or 16) ment going from 4.2.1 to 4.2.2 and brought these problems with it. Maybe there were some changes with MTP, because MTP is default "on"?
Anyway, everything works smoothly again and I am a happy puppy. Thanks guys !