I think it's what it says: Moodle is configured to use a proxy for web access. Moodle (from the web server not the browser) attempts to connect to the configured token endpoint but the proxy server is refusing to allow the connection replying with status 403 Forbidden.
Presumably Moodle has a web proxy configured so check that's configured as expected. Under the hood Moodle is using Curl to make this request so if you're familiar with this you can test connecting to the token endpoint from the Moodle server using the command curl to troubleshoot the settings.