Comment 17 for bug 1251345

There is a package version mismatch on the region controller and cluster controller due to different repos being installed.
On the RC:
Havana cloud archive is enabled, i.e. /etc/apt/sources.list.d/cloudarchive-havana.list:
deb http://ubuntu-cloud.archive.canonical.com/ubuntu precise-updates/havana main
deb-src http://ubuntu-cloud.archive.canonical.com/ubuntu precise-updates/havana main

This has upgraded the package:
python-kombu:amd64 (1.4.3-1, 2.5.12-0ubuntu2~cloud1)

On the CC:
There is no havana cloud archive enabled only the cloud-tools archive which means package:
python-kombu:amd64 (1.4.3-1)
is installed.

So enabling the havana cloud archive on the CC has fixed the issue:
add-apt-repository cloud-archive:havana
apt-get update
apt-get dist-upgrade

service maas-cluster-celery start