One (slightly) plausible theory ...
  • The game finds each world server by a DNS record.
  • The IPs of the servers changed when they were upgraded.
  • Unfortunately the DNS records for the servers have some crazy high TTL.
  • Your ISPs DNS cache smartly ignores SE broken settings
  • Your router/PC isn't that smart and keeps trying to connect to the wrong IP

In any case, rebooting my router fixed things for me.