Error 521意思

Error 521 is an HTTP status code that indicates that the web server is reachable, but that the web server is not responding. This error is often caused by a problem with the server's configuration, such as a misconfiguration of the SSL certificate or a problem with the server's hostname.

The 521 error code is defined in the HTTP protocol and is used by the server to indicate that it is not able to fulfill the request. When a client (e.g. a web browser) receives this error, it usually displays a message similar to "Web server is down" or "Web server is not responding".

If you are encountering this error, it is usually a good idea to check the server's configuration and make sure that all of the necessary settings are correct. You may also want to check the server's logs to see if there are any additional details that can help you troubleshoot the problem.