apt - Can't get rid of GPG-error: http://download.opensuse.org

I have been googling for hours trying to find a way to get rid of the annoying gpg-error referring to the link below.

W: GPG error: http://download.opensuse.org Release: The following signatures could not be verified because the public key is not available: NO_PUBKEY 9A5EED8CBCA00D90

I have tried:

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 9A5EED8CBCA00D90

but to no avail.

Google Translated error message:

The following signatures could not be verified because the public key is not available