Cloudflare failed to establish a TCP connection to the website hosted behind it. If Cloudflare does not get a SYN+ACK back within 15 seconds as part of the TCP handshake this error is returned.
Note right of Website: Behind Cloudflare
User ->> Cloudflare: Initial request
Cloudflare ->>+ Website: Initiates TCP connection
Cloudflare -->>+ Website: TCP connection fails
Cloudflare -->> User: Returns 522 error