Possible causes:

---

• 400 Bad Request: The 400 status code, or Bad Request error, means the HTTP request that was sent to the server has invalid syntax.
  
• 401 Unauthorized: The 401 status code, or an Unauthorized error, means that the user trying to access the resource has not been authenticated or has not been authenticated correctly.
  
• 403 Forbidden: The 403 status code, or a Forbidden error, means that the user made a valid request but the server is refusing to serve the request, due to a lack of permission to access the requested resource.
  
• File Permissions: 403 errors commonly occur when the user that is running the web server process does not have sufficient permissions to read the file that is being accessed.
  
• 404 Not Found: The 404 status code, or a Not Found error, means that the user is able to communicate with the server but it is unable to locate the requested file or resource.
  
• 500 Internal Server Error: The 500 status code, or Internal Server Error, means that server cannot process the request for an unknown reason. Sometimes this code will appear when more specific 5xx errors are more appropriate.
  
• 502 Bad Gateway: The 502 status code, or Bad Gateway error, means that the server is a gateway or proxy server, and it is not receiving a valid response from the backend servers that should actually fulfill the request.
  
• 503 Service Unavailable: The 503 status code, or Service Unavailable error, means that the server is overloaded or under maintenance. This error implies that the service should become available at some point.
  
• 504 Gateway Timeout: The 504 status code, or Gateway Timeout error, means that the server is a gateway or proxy server, and it is not receiving a response from the backend servers within the allowed time period.