Sentinel is in fact stating that it is running fine and the issue is with the masternode itself (funds moved out, genkey reused, etc) .. Like clockuniverse suggested, join mattermost for further diagnosis and help for which commands to utilize in order to nail down your issue as we can help you live there.

Edit: Specifically this: KeyError: 'Invalid Masternode Status, cannot continue.' states the issue is with the masternode, not sentinel