I have not tried backing up my Library and then reinstalling MC as I hope there is another explanation and solution. Does anyone have any ideas?
If you create a new backup (with the zone switch not working) and reinstall MC, it's a virtual certainty the behavior will be identical to what it is now after the reinstall.
Instead, restore from an existing backup (from amongst the ones MC automatically makes) from a date/time when you know zone switch was still working - you only need to tick the box for
Restore Settings. You can restore everything by ticking the
Restore Library and Playlists checkbox as well, but I don't think that is necessary. Bear in mind though, if you restore the entire library and you've made a lot of tagging updates between the "zones working" date and where you are today, you will lose all of that. That said, there are some ways to alleviate some of the tagging losses, but I won't bore you with those details unless it turns out you need that guidance.
***
Oh yeah… You might want to remove [solved] from the subject line, since at the moment your issues are only partially solved. Including that in the title will likely lead users users to skip reading your post. I think what I suggested above will solve your zone switch issue, but then again, it might not.