Some messages are not archived. They end up in the queue in the pool monitor, but keep failing. All with similar errors: The property Mentions is valid only for Exchange2015 or later versions. We are on office 365, so that shouldn't be a problem.
Please adjust EwsSupportedSchema registry key to "Exchange2015" in MAM Registries.
Open Registry editor on MAM server and change Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Metalogix\exchangePAM\EwsSupportedSchema
Change to value: Exchange2015
