I try to implement an #activitypub server in #Rust, have problems talking to #mastodon and would appreciate some help.

I try to send a follow action to Mastodon. The request is - as far as I can tell - signed according to https://docs.joinmastodon.org/spec/security/

The "keyId" of the signatrue is "https://pub.saar.social/user/achim/key". Mastodon calls this url as expected and my server returns: [...]