What does QNetworkReply :: ProtocolUnknownError mean?

Question

I am using an object QNetworkAccessManagerto host json data on a website.

when a response that is a class pointer QNetworkReplyin a slot function is processed , the value reply->error()is equal 301and the value reply->errorString()is equal Error downloading http://www.example.com/query - server replied: Service Unavailable.

I check the documentation QNetworkReply, a description of this error:

QNetworkReply :: ProtocolUnknownError | 301 | The network access API cannot complete the request because the protocol is unknown.

and I also know that this error is different from the http status 301.

and I add http://in front of the url.

Can someone give some information about this?