Sometimes things work out:
Took the levers apart with some fidgeting. Normally one could unscrew from the front after removing the bezel. In this case, I had to tilt the panel as a whole and found out that they used whatever they had at hand so way too long screws and two different sizes nuts.
Nevertheless: got it out and found:
On bottom right of the picture you see a metal piece in white plastic. This is the problem:
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That white plastic piece is supposed to look like the one on the top. It contains a rocker made of metal, pushed against by that metal rod which in turn is screwed in our out from the front next to the levers. By doing so, the rocker pushes a plastic rod against a slide. More pressure = more friction. No pressure = lever drops as much as the cables allow.
What happened: friction got less, screw was turned, not enough happened, screw turned more and more untill the plastic bottom broke, fell apart and dropped into the cable salad. Probably the slide got greasy (or even greased) so tightening the screw had no effect anyway.
All in all not a very smart design as the forces push the bottom of the plastic outward, there's no limiter before that happens. With a small metal stud - like casted in on the sides inside the plastic piece - this would have been prevented at no extra cost.
Got the part out, took it home and then discovered the rocker thingy. Glued the white piece back together with an enforcement clamp using 2K glue.
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Couldn't accept not having that rocker thingy though, so went back to the boat and.......
found it way down in a little corner!
Waiting for the glue to harden and then put it all back together tomorrow.
pushing/pulling the cables went quite smooth so I suspect that they were mounted to the wrong holes and thus not having enough leverage to move the other control on the same cable. But that's for tomorrow to figure out.