Sorry, sometimes you are so absorbed in a problem that you overlook the simplest things. Maybe this has happened to you at some point - that does not always have anything to do with laziness.
Anyway, thank you for the hint.
Well, I've now a general procedure to get Recalbox running on a Pimoroni Picade.
Because I think this could be a valueable information, I decided to finish this thread and to open a new and fresh one. So nobody has to read through all the posts in this thread to come to the solution.
Two years ago I bought the Pimoroni Picade (1st version) Arcade cabinet. It's slighty bigger than the current, 2nd version, has a 800x600 pixel display and a Pimoroni Picade PCB extension board - and I got Recalbox running without any troubles.
A few weeks ago I replaced it with the Pimoroni Picade (2nd version) Arcade cabinet. It's slightly smaller than the old, 1st version, has a 1024x768 pixel display and a new Picade X Hat extension board - and I didn't get it running at all!
The reason why Recalbox on the new Picade does not work is because the "Picade X Hat" extension board needs to have a driver installed. That's no problem with RetroPie, but it's NOT an easy thing with Recalbox, if it is even possible.
So I thougth I simply order the old Picade PCB and replace the new Picade X Hat. But I learned that even with the Picade PCB installed, I didn't get Recalbox running, because I didn't get the controlls (joystick and buttons) configured.
So I contacted the friendly guys from Pimoroni; and they had the solution for me - and for everyone else!:
It's the firmware of the Picade PCB which is now (obviousely) newer then the firmware shipped with the PCB 2 years ago. But Pimoroni provides the previous firmware on it's Github channel.
As soon I flashed the firmware "picade_original.hex" to my Picade PCB, all troubles vanished. Now it's again possible to configure the joystick and controlls/buttons within the Recalbox configuration dialog wizard.
The Picade PCB is an Arduino compatible board and fully corresponds to an Arduino Leonardo (ATmega32U4 AVR). The provided firmware is in Intel Hex format (*.hex) and can be flashed with "avrdude" quite easily. You will need a avrdude supported programmer (like the AVRISPmkII). I'm not sure if it's possible to flash a *.hex file within the Arduino IDE to the board.
The guy from Pimoroni additionally mentioned that the other provided firmware "picade_gamepad.hex" should work as well, because this firmware emulates a standard gamepad. I've never tried out this solution, but if anybody does, let me know how it works!
I hope I could describe the way to get Recalbox running on a (every!) Picade. Excuse me bad english, it's not my native language.
If anybode needs additional help, please contact me.