Since some weeks the displaylink driver never finds the third screen (laptop
integrated, external 1 + 2). Used to work for at least six monthes.

I tried everything, all repos offering evdi / displaylink. Both external
screens are connected to the Lenovo Hybrid dock. xrandr neither finds the third
screens.

If nobody else has been hit by this, then the cable might be broken, but how
can a cable lying on a desk break? 

There seems to be an issue: 

https://www.displaylink.org/forum/showthread.php?p=97631

https://bbs.archlinux.org/viewtopic.php?id=301305

I'm very motivated to help debugging, where to start?