For one, you need to upgrade to v1.17.2 and install the latest build of the OAuth2 plugin. v1.17 had quite a few bugs that were subsequently addressed in later versions.
Secondly, after doing the above you can delete the email and re-add it. If you run into the same issue then you'll need to check your logs (general server logs, webserver error logs, PHP error logs, MySQL/MariaDB error logs, osTicket System Logs, Browser Console logs, etc.) for any related errors.