This error is only related to Jira Server/Data Center.
If you can't log in - you try to log in, but when you click Login it starts logging in and after that nothing happens, then check the Excel 365 Connector logs and there is a 503 Error then it could be connected with::
invalid certificates on a Jira instance
that the firewall is blocking requests from our add-on (please check that the firewall accepts connections in the [server]:[port] format - because this is the format in which our add-on sends requests to Jira)