@Gargron I just received a mail saying someone started following me. I entered the link and the site said "CONNECTION FAILED".
After digging around I found it was caused because I wasn't logged in. After logging in, I reloaded the link saying connection failed, and everything works again.
I can not reproduce it again by logging out, though, so maybe you can check the logs of mastodon.social to see the bug? It happened one minute ago.