The magic a little ceramic 100n cap can do! thechpaul it worked! Thank you so much
For the record obcd I could reproduce the error with a simple C8 cable like this one:
with nothing on the other end! Weird huh!? Also the power usb cable has no filter inline. Im not sure about the power supply though. Its from a (expensive) Samsung tablet. I would assume it has some sort of protection but then again I tried a proper lab psu on 5V and the problem was there...
* Grounding the spare GNDs did not change the problem.
* Adding the cap on LCD's power did not change the problem. I had the buttons removed trying to simplify the setup.
* Doing both did the trick!
Im so releaved! It was a bugger!
Thanks again Paul!