Fungus causing my board to shortcircuit?
It seems possible that water has got in and some form of electrolysis has taken place and the white deposit you see that is remaining is some kind of salt (an electrolyte present in most water) that, with the presence of some moisture, will still act as a reasonably good conductor of electricity.

how to restore the functionality of this board

There's half a chance that if you removed the &quot;salt&quot; (or deposit) that the problem will be fixed. Then I suggest you lacquer the board but don't get the lacquer on the switches in case it seeps in. However, the switches may already be damaged internally but, luckily, they are common or garden switches and can be easily replaced.