Back here to let you know that with the software provided by Kakuro i managed to downgrade the firmware on the controller. This didn't work under windows 10 and also not under windows xp with a no-name usb midi adapter.
Luckily i have a Terratec EWS88 in stock, which is mentioned in the manual of the software. This card installed in a windows xp machine did the trick.
Still need to test with the real 802 and 402's, but the lab test with some simple desk speakers sounded like everything was in working order.
Couldn't find any evidence of a battery failure, after a longer (more than one day) power disconnection the device still starts with the same preset as when i turned it off. I opened up the casing of the device for a quick inspection of the insides, but couldn't find anything what looks like a battery.
Thanks for all the help!
(now hoping the buyer of the controller wants to sell his entero 4400 for a good price, it would be a nice combination)