Not sure when this has happened but when I try to update ubuntu (sudo apt-get update) I get the following error
Hit:1 http://gd.tuwien.ac.at/opsys/linux/ubuntu/archive zesty InRelease
Hit:2 http://gd.tuwien.ac.at/opsys/linux/ubuntu/archive zesty-updates InRelease
Hit:3 http://gd.tuwien.ac.at/opsys/linux/ubuntu/archive zesty-backports InRelease
Hit:4 http://security.ubuntu.com/ubuntu xenial-security InRelease
Ign:5 http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_17.04 InRelease
Hit:6 http://gd.tuwien.ac.at/opsys/linux/ubuntu/archive zesty-security InRelease
Err:7 http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_17.04 Release
404 Not Found
Hit:8 http://archive.canonical.com/ubuntu zesty InRelease
Hit:9 http://archive.canonical.com/ubuntu xenial InRelease
Reading package lists... Done
E: The repository 'http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_17.04 Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.