This gist seems to be not working anymore
Possible alternative solution https://unix.stackexchange.com/a/508728 ( not tested )
$> deb http://ftp.debian.org/debian jessie-backports main
-bash: deb: command not found
Open sources.list with sudo ( if available )
$> nano /etc/apt/sources.list
Paste the deb command in the end of file
deb http://ftp.debian.org/debian jessie-backports main
And them run
$> sudo apt-get update
@vuolter, Cool, maybe I'll try it when this happen again. ( although I hope never see this error again )
I made this gist to "remember" how to fix this error, because it was a really hard to find how to fix this