It works!
It works!
Great news!
Today I disassembled the machine completely and took out the motherboard. Then I re-seated all the RAM chips. Idid it like this: using a tiny screwdriver I carefully lifted the chip in its socket (only a few mm) first on one side ten on the other. After that I firmly pushed down all chips in their sockets again. For good measure I opened and closed the cpu socket as well (I have not seen this type of socket before - pins are straight out on all four sides).
After partial reassembly (with only the LCD controller, LCD and keyboard) I tested it. That worked ok, so I reassembled all of it.
I have now checked all the floppies, this took almost an hour.
Conclusion - the machine now works.