Thanks @Sineos for your summary and your Ideas, but I’m afraid the game for my Octopus seems to be over.
I’ll surely continue investigations but i think it is time to order a new board.
So, at this point one question comes up: Which board is recommended to replace the BTT Octopus v1.1?
Use the same Octopus again (simple solution, cheapest maybe) or head for a BTT Manta M8P (I stumbled across that in another topic here) which looks promising (will be more expensive, but what are the benefits)?
So, old problem, new question.