One of the main instances of Lemmy (lemmy.ml) is blocking 'kbinbot' user agent requests on their instance. Basically, they are blocking kbin instances. This is not a bug, but on purpose.
I couldn't find anything related in their source-code, so this is block is only present on lemmy.ml (or maybe some other Lemmy instances).
This is causing quite a lot of federation issues with Kbin instances. Resulting in various failed messages (from the messenger handler).
$ curl -I --user-agent "kbinbot" https://lemmy.ml HTTP/2 403 server: nginx date: Wed, 28 Jun 2023 18:35:27 GMT content-type: text/html content-length: 146 vary: Accept-Encoding $ curl -I --user-agent "notkbin" https://lemmy.ml HTTP/2 200 server: nginx date: Wed, 28 Jun 2023 18:35:42 GMT content-type: text/html; charset=utf-8 content-length: 163388 vary: Accept-Encoding x-powered-by: Express content-security-policy: default-src 'self'; manifest-src *; connect-src *; img-src * data:; script-src 'self' 'unsafe-inline' 'unsafe-eval'; style-src 'self' 'unsafe-inline'; form-action 'self'; base-uri 'self'; frame-src *; media-src * etag: W/"27e3c-LCqXylkqSkDOy3K+3w2TijtMn14" strict-transport-security: max-age=63072000 referrer-policy: same-origin x-content-type-options: nosniff x-frame-options: DENY x-xss-protection: 1; mode=block