Of course I don't know if this is a universal fix or just for my own specific AMS issue. So I would really appreciate if other AMS owners with the same problem try this fix and give a feedback here.
You need to attach the print with a small stripe of double sided tape!
As many other AMS owners, I had the very annoying problem that in 80 % of the time the filament jamms at the retraction on slot 3.
It doesn't matter which spool or what brand or kind of filament I had used, it was always the same. I printed several different fixes, changed the internal PTFE tube and double checked the motor unit. Nothing.
Then I noticed, that when I disconnect the long PTFE tube on the back of the printer, which heads into the hub on the other end, and press play on the display, the filament is pulled back without problems.
Getting curious about that, I moved the coupled PTFE tubes a bit away from the printer by hand when the AMS was about to chance the slots. For my surprise, the filament retraction worked every time I did that. So I designed a simple chock with a 45° angle to push the tubes a bit away from the printer and secured it with a small stripe of double sided tape.
And what should I say... I've printed several quick and dirty designed test cubes with many color changes, and now, after more than 100 successfully retractions on slot 3 without any issues, I'm quite sure my problem is gone. I don't know why, but it works.
Update: Ten days and several hundred filament changes on slot 3 later, and not a single retraction issue.