Error after upgrade to PE 2017.2.3

[Jonathan Gazeley]

Hi folks,

I just upgraded my PE installation from 2016.4 to 2017.2.3 and I've
started getting this error when I attempt to run the agent on the
newly-upgraded Puppet Server (monolithic install).

Error: Could not retrieve catalog from remote server: Error 500 on
SERVER: Server Error: Could not autoload puppet/type/firewall: Attempt
to redefine entity 'http://puppet.com/2016.1/runtime/type/firewall'. Set
at unknown location on node puppet-prod.resnet.bris.ac.uk
Warning: Not using cache on failed catalog
Error: Could not retrieve catalog; skipping run

I haven't changed anything else. Any ideas what could be wrong? At the
moment I've stopped puppet agent across my entire site so there is no
service-affecting outage, but I do not have control of my systems.

Thanks,
Jonathan