Comment 5 for bug 1730672

Thorsten - you can workaround a few different ways. For me, it's a pain because I move from a proxy-only to a non-proxy environment as I move my workstation around.

Workarounds:
1. set the http_proxy, https_proxy, ftp_proxy environment variables in root's environment, reboot.

2. prefix any apt command with the variables,ie

sudo http_proxy='http://[username]:[password]@[webproxy]:[port]' apt update
sudo http_proxy='http://[username]:[password]@[webproxy]:[port]' apt upgrade
sudo http_proxy='http://[username]:[password]@[webproxy]:[port]' apt install [package]

Hope that helps. Proxy issues have been appearing on and off in Ubuntu for a long time (9.x). They usually get sorted out eventually.

etc