Standard web browser and any application that uses a proxy can not get authorization from the proxy. I used a sniffer and saw that the proxy is sent only one of the first letter of the username and the domain (NTLM authentication). The old version of qt 4.4 is working properly and passes the username and domain entirely in the proxy.

p.s. I can not compile qt 6.2 browser itself, I downloaded it along with the SDK from the official site.