If you have Universe Sandbox 1 installed on your computer, it will be associated to the save files that Universe Sandbox ² creates. So if you double click on them in the browser, it will launch Universe Sandbox 1 and not be able to understand the format of the save from Universe Sandbox ². Which, gives you a blank screen.
To open a save from Universe Sandbox ², you need to launch the game and then use the home menu to select open simulation, and then go to the tab "My Sims" they should be listed there.
Though, I will mention that the saves are currently broken in Alpha 13.1 due, to some major code restructuring we're going through. We'll hopefully have the saves up and running again soon.
The warning you're seeing is from the free "demo" copy of Universe Sandbox 1 that is apparently on your computer.