Looks like mastodon.art is limiting federation with mastodon.social due to the high volume of spam accounts over there.
If you're looking for an instance similar to mastodon.social and still want to interact with folks on mastodon.art, consider joining mastodon.online. It’s invite-only, so you'll have to be invited if you want to join. Let me know if you want an invitation!
Edited 24d ago