Ok, just looked at zenstatus.log/zenprocess.log/zenpersnmp and this message is appearing in all of them:
013-06-24 09:30:18,341 WARNING zen.zenstatus: No service named 'EventService': ZenHub may be disconnected
2013-06-24 09:30:18,342 ERROR zen.maintenance: Maintenance failed. Message from hub: ZenHub is down
zenhub doesn't seem to show any issues and seems to be running fine.
--Rob