Make a full backup and try to install Openfire again.(or unzip it in a new-user folder and copy openfire.xml and the certificates - then make sure the server is stopped before starting the new one). It should start without errors.
Actually nothing can break if you kill Openfire and restart it. You could run it on a read-only filesystem after setup - only logs and database are written. Maybe Openfire did drop an UTF-8 setting in openfire.xml.