OK, I mark this as solved. The problem is that it seems to work in a different way than it is explained in the WIKI: If I touch and hold a controller, I do get the green ?? and can assign a MIDI CC channel, but using “Learn” is not working.
I cannot use MIDI learn anymore. Not sure what happened, but when I touch “Learn”, the yellow ?? appear, but moving any controller on the touchscreen does not make them turn green. I cannot see any errors in the logs. I tried with different plugins (Shiroverb, MVerb, ALO) and none of them is able to use MIDI learn.
I am using a Pi4 with a touchscreen connected to it and I used to be able to use MIDI learn with the touch screen. After I discovered that problem I ran software update, but the problem is still there.