522Connection Timed Out

When you see it

Different from 521: there the connection was refused immediately, here it hung. That usually means a firewall silently dropping packets, an overloaded origin too busy to accept connections, or a routing problem between the two.

The distinction matters when debugging, because a refusal and a silence point at different things. Refused means something answered and said no; silence usually means a firewall rule dropping traffic without a reply.