I kept getting this error in my watchdog message when testing the MCP module:
Path: /mcp/post. Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException: The used authentication method is not allowed on this route. in Drupal\Core\EventSubscriber\AuthenticationSubscriber->onExceptionAccessDenied() (line 135 of /var/www/html/docroot/core/lib/Drupal/Core/EventSubscriber/AuthenticationSubscriber.php).
The error message could be improved by specifying which authentication method was involved. In this case, it was basic_auth. Knowing that would have saved me about an hour when debugging.
Proposed help text: The authentication method %name is not allowed on this route.
We could also consider specifying which authentication methods are allowed instead.
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.