Since 5.4.2, cannot install Qt with online installer behind a proxy

Trying to use MaintenanceTool behind an Enterprise proxy. The problem is that it returns to the proxy username/password prompt again and again, and does not accept the credentials given. I know the proxy itself functions because we use it regularly (and I'm using it now to access this bug reporter).

Strangely, when I specify my Qt account/password at the start of the install/update process, it accepts the proxy credentials then and logs in. However after that, when it is trying to access the repositories, it then does not accept the credentials and goes back to the prompt over and over.

Attached is a text file of the MaintenanceTool --verbose output for this.