update:
Thomann accepted the signed document which I had from the original buyer (stating he would give me 2nd hand users warranty) and asked me to ship in the faulty unit.
Then they would inspect it and depending on their assessment would either handle it as a warranty claim, or bill me …
… totally (!!!) ignoring, that I already talked with Elektron and they said it is a warranty issue, and ignoring a video link which I supplied showing the broken button
damn!
I had it with thomann.de FOR SURE
(bought goods for over 2000 Euro from that online retailer and get treated like that)