Following an ActivityPub Actor

What is the most propably reason for error 403 when a mastodon-user tries to find discourse-actor?

On Mastodon that handle finds nothing, and Discourse gives this:

GET request received by https://foorumi.katiska.eu/ap/actor/2a8f2bdb5a547366558a0906d6bf6a5c failed: 403 Forbidden

And same for /inbox