online installer doesn't work after changing proxy settings

In the office I'm behind a corporate proxy, but I have cntlm utility running as well as HTTP(S)_PROXY envvars defined.

When I start the installer, it is "Performing license pre-conditions check" indefinitely. Then I go to proxy settings, enter the address where cntlm is listening, and after saving settings the installer is simply stuck, see video attached. After restarting the installer, proxy settings are not restored (QTIFW-3135).

Hence, currently I'm simply unable to use the online installer behind proxy. QTIFW-3185 would have helped though.

In the verbose output I see this:

[24907] Warning: QIODevice::read (QNetworkReplyHttpImpl): device not open
[24908] Warning: Network error: [ QNetworkReply::OperationCanceledError ] "Failed to connect to server. Check your network connection and try again."
[24908] Warning: Failed to connect to server. Check your network connection and try again.
[45285] Warning: Network error: [ QNetworkReply::HostNotFoundError ] "Failed to connect to server. Check your network connection and try again."
[45285] Warning: Failed to connect to server. Check your network connection and try again.
[65565] Warning: Network error: [ QNetworkReply::HostNotFoundError ] "Failed to connect to server. Check your network connection and try again."
[65565] Warning: Failed to connect to server. Check your network connection and try again.

P.S. Using proxy address without http:// prefix also doesn't help.