Puppet/Hiera: Error 400 on SERVER: undefined method `empty_answer' for Hiera::Backend:Module

Question

I had a puppet master running for quite some time. After a restart of the process recently, I started getting:

    # puppet agent -t
    info: Retrieving plugin
    info: Loading facts in /var/lib/puppet/lib/facter/mountpoints.rb
    info: Loading facts in /var/lib/puppet/lib/facter/pe_version.rb
    info: Loading facts in /var/lib/puppet/lib/facter/concat_basedir.rb
    info: Loading facts in /var/lib/puppet/lib/facter/disks.rb
    info: Loading facts in /var/lib/puppet/lib/facter/root_home.rb
    info: Loading facts in /var/lib/puppet/lib/facter/puppet_vardir.rb
    err: Could not retrieve catalog from remote server: Error 400 on SERVER: undefined method `empty_answer' for Hiera::Backend:Module at /etc/puppet/environments/production/modules/sshauthkeys/manifests/helper.pp:2

If I run

    hiera -c /etc/puppet/hiera.yaml <key>

it works as expected. Any idea what this could be?

I'm running puppet 2.7, hiera 1.2.1 and hiera-puppet 1.0.0

Kind regards,

Kristian