[ciscospirit]

have you tried using ISTA and follow the fault guided troubleshooting test plan?

yes, but sadly ISTA just say: "NVE-Steuergerät erneuern"
there is no testplan or other troubleshooting or other error detail available.

searched also hours on the internet and can't find any help for this error code.

Sadly? are you happy it tells you it says to replace the control unit? seems very straight forward. What other help are you looking for? Are you going to take the control unit apart to check the circuit board for capacitor, mosfet, resistor failures?

it sounds like a very "generic" error message and i am just wondering why it works sometimes and sometimes not.
so i was not sure, if replacement is the only thing how i can fix it. Also replacement is a pain to retrofit another ECU.

so i thought maybe it could be some software related problem i can solve on my own. 
Maybe wrong FDL/VO Coding or whatever.

but maybe you are right and i picked a broken ECU out there :/