Thats the same error at my first try to start the Desktop app. (Arch Linux - 64 bit Gnome 3.24.2)
The output of 'env | grep -i proxy' gave me exactly my working proxy-settings. It is an Auth-proxy like: http://username:password@proxy-ip.com:8080/
Do you know what software powers your proxy? I'd love to try and reproduce that issue.
