Well, 4 months :o after delivering the Helix to an authorized service center here on the canarian islands, i got it finally back today.
I already discovered a transistor in the guitar input section, wich seemd to be physically damaged, and the service center confirmed it really was. Aditionally, they said a OP amp and a diode were broken and had to be replaced. The price for the repair of only 30.- € helped a bit to forget about the 4 months of waiting.
Just made a quick check - it's living again.
A thing i'm not absolutely sure about: I always had a problem with note separation, means playing for exemple a power chord and adding a single note to it - it nearly dissapeared in the mud. In comparision, my other (cheap) devies like Nux MG300, Zoom G1on or actual the ToneX Plug seemd to do this topic better. After the repair, it's much better, so maybe a problem on this changed components existed all the time.