Main causes

Incorrect or missing proxy credentials configured in the client's system or application.
The proxy server settings have been changed, requiring new authentication details that have not been updated on the client side.
The proxy server may be misconfigured, experiencing issues, incorrectly denying access, or failing to relay proper authentication protocols.
Network changes or security policies recently implemented that require additional authentication steps or updates to the existing proxy configuration.

# Request example 
curl -X POST https://sandbox.oxylabs.io/ -H 'Accept: */*;q=0.8' -H 'Accept-Encoding: br' -H 'User-Agent: AppleWebKit/537.36'

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Common fixes

Verify your proxy server settings to ensure they are correct and active.
Check the credentials used for authentication with the proxy server; update them if they are incorrect or expired.
Ensure your network configuration allows for proxy authentication, adjusting settings if necessary.
Contact your network administrator to confirm if any specific requirements or configurations are needed for proxy access.

# Response example HTTP/1.1 407 Proxy Authentication Required Content-Type: text/html Proxy-Authenticate: Basic realm="Access to internal site" Proxy Authentication Required

Proxy Authentication Required

You must authenticate with a proxy server to access this resource.

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Common scraping HTTP error codes

401 Unauthorized	403 Forbidden	404 Not found	405 Method not allowed	406 Not acceptable
407 Proxy authentication required	408 Request timeout	409 Conflict	410 Gone	411 Length required
412 Precondition failed	413 Payload too large	414 Uri too long	415 Unsupported media type	416 Range not satisfiable
417 Expectation failed	421 Misdirected request	422 Unprocessable entity	423 Locked	424 Failed dependency
426 Upgrade required	428 Precondition required	429 Too many requests	431 Request header fields too large	444 Connection: close
451 Unavailable for legal reasons	499 Client closed request	501 Not implemented	503 Service unavailable	505 Http version not supported
506 Variant also negotiates	507 Insufficient storage	508 Loop detected	510 Not extended	511 Network authentication required