I am having the same problem. The "Data Model Viewer" opens fine but the "Data Manager" gives me the error that the app name already exists. Not sure what that error is telling me as I am trying to edit only one app.
It looks like this error was a red herring in my case. The data was already loaded so I did not think I would need the VPN connected for Data Manager to work. Looks like Data Model will work without a connection to the source data but Data Manager will fail if the connection is not active.