Unable to follow account

I've got an issue with one Wordpress site that runs the AP plugin, despite not having the same issue on three other Wordpress sites that run the plugin. I receive an error when trying to follow @ethicalrevolution@climatejustice.social (my personal mastodon account) from @theindiebeat@theindiebeat.fm (my Wordpress ActivityPub blog profile).

The climatejustice.social instance doesn't appear to have blocked theindiebeat.fm because I am able to see the @theindiebeat@theindiebeat.fm from my personal Mastodon account.

Any ideas why this may be? Here is the error:

**Unable to follow account “@ethicalrevolution@climatejustice.social”. Please verify the account exists and try again.**

Start by reproducing the failed follow from @theindiebeat@theindiebeat.fm to @ethicalrevolution@climatejustice.social and compare it with the three WordPress sites that work. Trace the ActivityPub follow request and its error response, then verify the fix by repeating the follow attempt and confirming the account can be followed successfully.

Written by the indexing model from the issue text.