Seems to be a regression. It worked in 6.5.2 but not anymore in 6.5.6. Nevertheless, problem is gone by 6.7.2 (or some commits to 6.5.6 that break things are not merged into 6.7.2 yet?)

Anyway, build the reproducer with 6.5.6, and after "WaitForReplyFinish" the error will be "QNetworkReply::OperationCanceledError". That means within "WaitForReplyFinish", "reply" is aborted due to timeout. But with 6.5.2 (or 6.7.2), the error is "QNetworkReply::AuthenticationRequiredError" which means authentication finishes prperly in time (just nothing is granted).