-What is Error 400?
Error 400: Bad Request means; the request is incorrect.
-What is Error 401?
Error 401: Unauthorized means; the client does not have the required privileges to access the site.
-What is Error 403?
Error 403: A 403 Forbidden Error occurs when you do not have permission to access a web page or something else on a web server.
-What is Error 501?
Error 501: Not Implemented means; the server does not support the service type or the called protocol.
-What is Error 404?
Error 404: Not Found means; the requested resource no longer exists or has been moved, or the address may be misspelled.
-What is Error 500?
The server encountered an unexpected condition that prevented it from fulfilling the request.
-What is Error 503?
The Web server (running the Web site) is currently unable to handle the HTTP request due to a temporary overloading or maintenance of the server.
-What is Error 520?
Error 520 is essentially a catch-all response when something unexpected happens or when the origin server incorrectly interprets or does not tolerate a request due to a protocol violation or an empty response.
What is Error 521?
Error 521 occurs because the origin web server refuses a connection from Cloudflare. More specifically, Cloudflare tried to connect to your origin server on port 80 or 443 but received a connection refused error.
Source: Google
