5xx Status Codes
Internal Server Error

500 (Internal Server Error)

The server encountered an unexpected error which prevented it from fulfilling the request.

501 (Not Implemented)

The server does not understand or support the method required to fulfill the request.

502 (Bad Gateway)

The server, while acting as a proxy (that is the middle server between you and a second server), received an invalid response from the upstream (second) server it accessed in attempting to fulfill the request.

503 (Service Unavailable)

The server you are accessing is unavailable, most likely it is rejecting requests due to reaching the maximum number of allowed concurrent connections.

504 (Gateway Timeout)

The server, while acting as a proxy, did not receive a timely response from the upstream server and has timed out.

505 (HTTP Version Not Supported)

The server does not support the HTTP protocol version that was used in the request message.