Not sure if it is because it's hosted on PieFed, although that shouldn't matter, but trying to subscribe to !youdontmattergiveup@piefed.social gives a message saying I've unsubscribed. It also doesn't show any posts although there are some on the host instance (that's probably a federation issue though, nothing to do with Connect).
Just tried again with the latest version. It looks like the community information page, tapping "subscribe" works. However the Subscribe option in the three dots menu gives that error. I'm not sure about the new "join" button - that certainly looked like it worked, but refreshing put it back to Join so I'm not sure.