I have not found an answer to this but upon investigation of the source code I did discover that in the file issuers.php this is the line which fails and results in the error we are all experiencing.
So what is failing is the function that is connecting the "system_account" to the $issuer (Facebook).if (core\oauth2\api::connect_system_account($issuer, new moodle_url('/admin/tool/oauth2/issuers.php', $params)))