Well, I've never seen this one before.

Can you copy-paste the output of `env | grep -i proxy` ? (Just run it in gnome-terminal, select, Ctrl+Shift+C to copy, paste it here)

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/