This applies to all Datto Siris, Alto, and DNAS devices.
VNC processes fail to run properly resulting in a grey or "X'd out" VNC session.
Try creating a new VNC session via the Partner Portal. See this article for instructions: How To Connect To A Datto Device Via VNC Locally And Remotely
If this does not resolve the issue, please contact Technical Support for assistance in resetting the VNC process.