That is even stranger and needs further investigation.
The filename is shown top left as ‘Untitled’ which probably means no file at all and the error messages are misleading.
Use Tools > Preferences… and open the Startup tab.
Please copy the entire contents of the Default Startup File: box into your next reply by using Ctrl+A, Ctrl+C and then Ctrl+V.
Is that file path the correct one for your project?
Please post a screenshot of the Project Window with the green tick and including all the buttons.
Confirm that clicking the Open Project button successfully opens that project.
Try using the More Tasks… button and select Samples > Reset Sample Project.
Does that successfully create a Family Historian Sample Project that you can open?
Presumably, you are running FH v7.0.8 from the same Windows PC Account Username as you ran FH v6.2.7.
Have there been any significant changes to your PC system between running FH v6.2.7 and running FH v7.0.8?