The Addon fields saving issue is mainly due to following reasons:-
Check you have polylang plugin installed and kept unconfigured without selecting the languages .The above issue may occur if you have installed and activated polylang plugin and kept it without configuring.
You need to either remove the polylang plugin or you need to fully configure it to resolve the issue associated with saving Addon fields.
If you have questions about our plugin(s), are experiencing issues with any of our plugin