A jenkins server which was communicating properly with gerrit, suddenly stops reporting both "Build Started" and build success or failure back to gerrit.

In an attempt to resolve, jenkins is restarted. Upon restart, the problem persists and this error message is seen in the jenkins log -

Apr 03, 2015 10:27:34 AM com.sonyericsson.hudson.plugins.gerrit.trigger.hudsontrigger.actions.manual.ManualTriggerAction getServerConfig
SEVERE: Could not find server null

While the jenkins server is in this state, the gerrit server status shows as green under https://myjenkins/gerrit-trigger/ and the test connection button under https://myjenkins/gerrit-trigger/server/defaultServer/ reports success.

Also during this failure, the retrigger links vanish from gerrit verifier builds. Screenshot attached.

The problem went away after a second jenkins restart. After this restart, new gerrit builds show the retrigger links, but one which were run during the period of failure still don't show them.