Data from such a repository can't be authenticated

[JokerGermany]

When I try to update (sudo apt-get update) Ubuntu 16.04 on any pc who has cairo dock installed, i get the following error:

W: The repository 'http://ppa.launchpad.net/cairo-dock-team/ppa/ubuntu xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://ppa.launchpad.net/cairo-dock-team/ppa/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
E: Einige Indexdateien konnten nicht heruntergeladen werden. Sie wurden ignoriert oder alte an ihrer Stelle benutzt.

I added the ppa this way:
echo "deb http://ppa.launchpad.net/cairo-dock-team/ppa/ubuntu $(lsb_release -sc) main # Cairo-Dock-PPA" | sudo tee -a /etc/apt/sources.list
     sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E80D6BF5

The error is really anoying because the update Error throw an error that I should check my Internet Connection...

[Ejaz Rasool (er-usmani)]

I'm getting the exact same issue on same Ubuntu version (16.04)

[Denis Leskovar (leskovar-denis)]

same issue in Ubuntu 16.04

[AC7NJ (ac7nj)]

Same issue for me along with multiple other problems logout not working, Graphics display not propagating until I mouse over even then its not permanent fix.
Maybe we can start with this: screen display problem

lspci -nnk | grep VGA -A1
00:02.0 VGA compatible controller [0300]: Intel Corporation Core Processor Integrated Graphics Controller [8086:0046] (rev 18)
 Subsystem: Dell Core Processor Integrated Graphics Controller [1028:0457]