I tried another adapter (12V 400mA). It didn't help.

Accidentally I have ordered another RU990 with a dock, which I will get within a week. I think I will try that one alone, to see if that works. If so, I will try one of my older Pronto's on the new dock, if that works, go over to the other two. If it doesn't I will go back to the new RU990 and see if it is still okay. And so on.

A main issue in two circuits has a very low probability. (And the circuits are protected.)

Probably I did something stupid. But what? I don't know.
USB works fine, by the way.

Anyway I will come back here and let you know.