If you received an error while making a request, visit Residential Proxies documentation. Here you will find a dedicated section - Response codes - with all the error codes explained. You can also find them listed below.
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400 Bad Request - Proxy server can return this error code if the request did not contain a host to connect to or there was a generic error when parsing HTTP request. Make sure your request is correctly formed and make sure to include URL in the request then try again.

407 Proxy Authentication Required - Request lacks proxy authentication information or username or password is invalid. Include Proxy-Authorization header in your request and make sure your username and password are correctly formed and then try again.

500 Internal Server Error - Proxy server has encountered an internal error. Retry request at a later time.

502 Bad Gateway - Proxy server received an invalid response from the upstream server. Retry request.
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Response Code 502 signifies that the IP assigned to your session ID is no longer available. If you encounter this error, there are two ways to work around it. The first is to wait for one minute and the system will automatically assign a new IP address to your session ID. Another approach is to simply switch to a new session ID (i.e. change the sessid parameter) – this way you will receive a new IP address.

522 Timeout - Proxy server did not receive a response from the upstream server in time. Retry request.

525 No Exit Found - Custom HTTP status code - this means proxy was unable to find an exit node which satisfies the request. Change request filter parameters or try again at a later time.