Thank you Tellie, great to have you involved here!
So I have checked according to your recommendations:
I am quite convinced to have a decent seating of the end caps. I had always banged them in by hand and now I have additionally used a piece of wood as recommended. Tightening the rings one feels a well defined final position.
Additionally, to make sure it is not a crack in the end ring, I have swapped the two end rings.
The leakage is still there, water squirting out from the thread between vessel and ring on the inlet side, even with the other end ring and with a, I think, nicely seated end cap.
So I have checked the membrane vessel for cracks. Its translucent in the thread area which is why I have "x-rayed" it with a 1200 lumen spot light. There is nothing unusual visible - all nicely homogeneous, no strange reflections or so. It should be ok than, right?
So what can it be? The o-ring I had changed already and as this did not help I have put some teflon tape underneeth to make it a bit thicker. I can certainly take this off again. As said I have changed the end piece o-ring and the end-piece itself looks very sturdy and nice. It can't be that, right?
I am asking myself what can have caused the problem. There was nothing unusual as far as I remember, no shocks or so. (even though we have two kids
on board and you never know what they have done...)
PS: I thought it is too late now to call you... Thank you for the offer!