We've also had this issue recently as well, again with E600 APs.
Our solution was to first on board the AP to an on-premise version that supported the current firmware version, update the firmware and finally move the devices over to the cloud using this method: http://community.cambiumnetworks.com/t5/cnPilot-E-Series-Enterprise-APs/Migrating-cnPilot-E-Series-Access-Points-from-cnMaestro-Cloud-to/m-p/108449#M128