This could happen with minor updates (i.e. trying to open an SDG file in Strater 5.0 that was saved in version 5.1), or with major upgrades (i.e. trying to open an SDG file in Strater 4 that was saved in Strater 5).
For minor updates, File | Online | Check for Update command to update your version of Strater and then try opening the file.
For major upgrades, follow the instructions here to purchase the newer version at the discounted upgrade rate.
Updated August 15, 2018