Can't add PPA on Ubuntu 15.10 - user or team does not exist [duplicate]

You need to export environment variable first. Open a terminal and run:

export http_proxy="http://username:password@proxy:port/"
export https_proxy="https://username:password@proxy:port/"

Now add the PPA:

sudo -E add-apt-repository ppa:git-core/ppa

You will get a prompt:

➜  ~ sudo -E apt-add-repository ppa:git-core/ppa
 The most current stable version of Git for Ubuntu.

For release candidates, go to https://launchpad.net/~git-core/+archive/candidate .
 More info: https://launchpad.net/~git-core/+archive/ubuntu/ppa
Press [ENTER] to continue or ctrl-c to cancel adding it

Press Enter and you will be able to install from a PPA.

OP's answer

I found the problem, it was really related to the proxy-server but with a different background. Our server uses authentication which was not set up here. After disabling it, the problem went away. The strange think is the misleading error message. So hopefully this will read somebody in the future with the same problem.