520: Unknown error
It causes generally:
- Connection Reset
- Headers than the header size limit (more than 8KB)
- Empty response Source
- Invalid HTTP response
- Lack of response HTTP response headers
521 - Web server is down
the source server rejected the cloud server connection
522-- connection timeout
No cloud server and the source server to complete the TCP handshake
523-- source is unreachable
Cloud server can not reach the original server, DNS record source server may be incorrect
524 - Timeout
The cloud server can complete the TCP handshake, but did not receive an HTTP response
525 - SSL handshake failure
The cloud server can not complete with the original server's SSL / TLS handshake
526- invalid SSL certificate
Cloud server can not verify the original server's SSL certificate
444-- no response
The server does not return the response, and closed the connection
495 - SSL certificate error
Return extended error code 400 when a client provides a valid certificate
496-- require SSL authentication
The client needs to provide SSL certificate, the client did not provide a return error code 400 extension
497 - HTTP request to the HTTPS port
Return extended error code 400 when a client sends a HTTP request to the HTTPS port
499-- client close request
Before the server sends the client response closed the connection
440-- Login Timeout
The client session has expired, you must log in again
449-- Retry
The client does not provide the required information, the server does not respond correctly
451 - Redirect
In using the Exchange ActiveSync when can not access the user's mailbox, you need to re-run to find a more appropriate server HTTP 自动发现
Source: https://blog.csdn.net/tan6600/article/details/51584087