I have to apology but you were right in first place.
Well the cable was plugged in the right direction but as you mentioned it does not always suit for 100%. I unplugged the cable, plugged it in again. And I dangled the cable a bit, suddenly it worked.... then I captured some seconds. Everything went fine. I wanted to rearrange the iMac back to the place it stood before on the desk. You know, we know, everybody know the slots are on the back...
#sigh In the end the signal got lost again. I tried it again and again no lucky me. I did not manage to get the signal back again. :(
Silly that such an expensive thing like an iMac '27 does show a behaviour like this. Would you propose I should cut some of the plastik on one end of the tb cable? So it would better fit in the slot? I know it costs 49 bucks but when this will prevent it from not showing the signal... I would do it. I have managed to do this with other cables for video or audio before.
