ERROR: Couldn't find any revision to build. Verify the repository and branch configuration for this job.

After an upgrade, Jenkins can't checkout the source code any more:

Retriggered by user brian for Gerrit:
Building in workspace /srv/hudson/jobs/karaage/workspace
Fetching changes from the remote Git repository
Fetching upstream changes from https://code.vpac.org/gerrit/karaage
ERROR: Couldn't find any revision to build. Verify the repository and branch configuration for this job.
[SLOCCount] Skipping results publication since the build is not successful
Sending e-mails to: brian@vpac.org
Finished: FAILURE

I am using the documented configuration: "If this does not work for you set Refspec to refs/changes/:refs/changes/ and 'Branches to build' to $GERRIT_REFSPEC." as at https://wiki.jenkins-ci.org/display/JENKINS/Gerrit+Trigger

As I said, this was working fine until I upgraded Jenkins and plugins.

Some more debugging information as what exactly failed would be really useful.