We have a virtual machine which uses two custom repositories, first is Mono Xamarin, I followed the official guide on installation and it works.
However we have custom repository created with Aptly which is signed with GPG as well. When I add this repository to the list and add our public key (via
apt-key add my_key.asc), the
apt-get update says
Failed to fetch https://our-repo/debian/dists/jessie/main/binary-amd64/Packages gnutls_handshake() failed: Public key signature verification has failed.
I am really not sure why because it works on all other machines. I am 100% sure that the key is correct and there is no difference to other machines except adding Xamarin repo as well. Additionally we handle this with Puppet so there really should be no difference compared to other VMs.