Have you tried a packet capture with Wireshark or tcpdump to see what it’s doing? It might give better clues than a general error message.

I’m working on adding ActivityPub to my Hugo blog right now. I support RSS, but I figured AP support means that you can get it into your Mastodon feed or even Lemmy feed making it easy to follow. Additionally, commenting (assuming it doesn’t get taken over by spammers.)