I had to add "localhost" to the webproxy bypass list of the VBR server:
netsh winhttp set proxy "<YourProxyURL>:<ProxyPort>" "<local>; *.domain.com; *.domain2.com;"
we found in the logs, that veeam is trying to connect to https://localhost:20443 and https://localhost:6172 and because <local> was not in the webproxy bypass list, veeam tried to connect to localhost addresses via webproxy
Oh, sweet! Glad that you were able to resolve it with the help of the support team (though from the 2nd attempt only). Thanks for sharing the solution with the Community.