522 Connection Timed Out
Meaning
A Cloudflare-specific status indicating that the proxy did not receive a timely response from the origin server. It is essentially a timeout between Cloudflare and the origin.
When to Use
When Cloudflare can connect to the origin server but the server takes too long to respond to the request. This is similar to a 504 but specifically in the context of Cloudflare communicating with the origin.
Example
If the origin server is overwhelmed or slow and doesn't respond to Cloudflare's request within Cloudflare’s timeout window, Cloudflare returns a 522 Connection Timed Out to the client to signal the upstream timeout.
When Not to Use
Do not use directly on origin servers. This is meant for proxies/gateways. If you are implementing a proxy or load balancer and want a similar concept, you might use 504 Gateway Timeout. 522 is part of Cloudflare’s nomenclature for timeouts.
Source
Unofficial (Cloudflare)