An update: I bought a 50W soldering station and with that, I was able to desolder the fuse and the VDR.
After I replaced both, the switch came back to life, just as you predicted
Everything is OK, except that the relais doesn't switch. I have desoldered that one too and found that it's OK. It switches at 12V/36mA.
The 12V contacts on the prints are still shorted, so I expect the diode D9 to be the culprit.
I only have a simple multimeter to measure things, but I would expect the diode to show resistance at least one-way. (It shows no resistance on my multimeter, no matter how I connect the black
and red probes)
What do you think?