@julian I think I resolved the issue partially. When I initially setup nodebb, the canonical url was set to http instead of https, I didn't change that until *after* my initial attempts to federate with my own Mastodon, ActivityPub.space and lemmy communities. http format was still in my Mastodon cache, so there was a mismatch when subsequent attempts were made.
Clearing the cache for my board from mastodon allowed me to follow my nodebb account from my Mastodon account successfully.