433 packages, impressive :) I’m stuck on 474 while keeping a working environment where I can do my things nicely. And that doesn’t count some hand compiled/written programs I have. Also, 175MiB of memory! I used to boot at around 400MB into my WM but over time it has gone up to a fat 600MB without changing anything :| Just nice to see someone going for a minimal system.
It is possible that the previous owner flashed firmware that doesn’t bind that key to anything. So the first thing to try would be flashing firmware that does bind the key. If that doesn’t work, the switch might be the problem. You could check on the back of the pcb if the soldering looks any different from the other keys. Even if it doesn’t you could reflow the key. If that doesn’t work you can unsolder the key and pull it out, open it up to see if anything is messed up like the contact leaf. You could try a different switch in that spot. If you put in a fresh key that works in the old spot and it still doesn’t work it might be the pcb. Maybe you need to reflow or replace the diode. If that doesn’t work it might be the contact pads on the pcb for the switch or the diode. When unsoldered and with the solder removed you should see a metallic ring around where the switch pin goes. If that is (partially) missing it will be trouble. It could also be the ‘wire’ that is etched into the pcb that goes from the pad to the controller. Either fixing the pad or jumping the wire is a bit more advanced (and a pain in the ass). I don’t have experience with that. Hopefully the problem is earlier in the chain. Good luck!