Error 401: "Host violates local access rules" when using a shared subdomain instance

[csolisr]

How to replicate:

• Install Nextcloud and this plugin on https://cloud.example.net
• Install Mastodon or similar on https://social.example.net
• Configure this plugin to have your instance point to the latter, then click on "Connect to Mastodon"
• The attempt will fail with the following error message: Failed to create Mastodon OAuth app: {"error":"Host violates local access rules"}
• Configure this plugin to have your instance point to any other public server, then click on "Connect to Mastodon"
• The attempt will be successful and the popup to log in will appear

[julien-nc]

Have you tried to set 'allow_local_remote_servers' => true, in your Nextcloud's config.php?

[julien-nc]

By default, Nextcloud refuses to connect to domains considered as local from the server's perspective.