W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04 Release: The following signatures were invalid: KEYEXPIRED 1496576244
W: Failed to fetch http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04/Release.gpg The following signatures were invalid: KEYEXPIRED 1496576244
The key from here gives the error
On ubuntu 16.04. Cheers!