@TinCanoftheSea
What you are currently running is the current (but old - regardless) stable version 2.5.22294.
The error message that you encounter indicates that the savegame has been created with one of the hundreds of newer versions (the latest version is 2.6.14580 as of now) that have been released since the latest “stable”.
I suppose that a version 2.6.xxxxx has been involved when saving the game. 2.6 games are most likely incompatible with games of older engines, depending on what your “old” engine version actually had been.
Now to open the savegame you should try and install a current pre-release version.
You don’t need to remove “old” TripleA installation(s). You can run as much versions as you want side by side. Just make sure to install each in a different installation directory.
Good luck.