Virtual standby export failure occur after upgrading Core to 18.104.22.168 (for a DL appliance) and having also upgraded the agents as well.
Dell Core has reported the Error event "Failed to enqueue continuous export": Date/Time: 12/30/2016 14:01:04 +00:00 Failed to enqueue continuous export of 'agent' to 'VMhost' at 'd:\'. Reason: Cannot start the export for unprotected volumes An unexpected error occurred An argument was not valid in code located in or called by 'CanStartExportValidator.Validate' --- About this event: Failed to enqueue continuous export of a recovery point to a virtual machine https://core:8006/apprecovery/admin/
This issue is due to a couple of reasons:
A new volume is either removed or added to the existing virtual standby which requires that the existing one be recreated - for those affected agents.
The result of a bad upgrade.