AWD2;
That error means that at some point when loading the checksum value saved for the data failed to match. Unfortunately, there isn't any way to determine exactly what changed, or if the problem may have been just with that load rather than the save itself. If it is a problem with the save, there is no way to tell if or how serious it may be (a developer build of the game would catch certain types of issues, but may or may be able to identify the exact problem).
If you are playing the Enhanced Edition, MooseEdit was not updated for that (it can only open/edit Classic saves).
Copying the save shouldn't cause a problem. Overwriting the backup shouldn't either, though making new copies each time (perhaps to a folder named for the date, for an end-of-session backup) would be safer and more reliable.
If this affects your backup save, as well, then you can either keep going and hope that if there is a problem it isn't serious, or you can start over in tactician (depending on how far you are into the game).
caninelegion;
Are you shutting down all non-essential programs (especially anti-virus) before starting the game? A real time scan could cause intermittent problems with the save process.
Try exiting out of the Steam client completely, and starting the game directly from the '..\SteamApps\common\Divinity Original Sin Enhanced Edition\Shipping\EoCApp.exe' program file, by right clicking and running as administrator.
If this happens frequently enough that it is easy to reproduce, try creating a new Windows administrator user account (possibly with the My Documents folder on a different drive, if available), switch to that account and try starting the game from there, directly from the executable.
