Please try to remember that I have used this plugin once to actually generate a cert. I really have no idea what certbot does with old certs and how your proxy mess works. I guess you could try removing the cert lines from nginx configs to get it to start or at least making sure they exist. I guess you could also remove cert references in the database and then regenerate the nginx configs with omv-mkconf nginx and restart it.
to not destroy my new cert?
If it doesn't work, why are you worried about keeping it? Can't you just generate a new one?