How far back is your most recent save without the tampering or corruption error? The warning means that at some point the checksum value when loading a save did not match the value created when the save was made. This means either something became corrupt or there was a problem with the loading process, but there is no way to tell just from that if that will cause a problem or how serious it may be.
Once this check fails a flag is set in the save, so any saves made after that point will always give this warning.
If you did not resave after getting the warning, try browsing to the '..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\LevelCache' folder and delete the contents. A corrupt file there can cause problems when loading, including triggering the corruption warning. If deleting the contents eliminates the warning when loading the save again, the file itself is fine and the previous warning due to a problem when loading.
Though very rare, a corrupt screenshot can also trigger this error, even when the save data is ok. Try browsing to the save folder and move, rename or delete the png screenshot file (especially if it gives an error trying to open it in an image viewer).
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\PlayerProfiles\<ProfileName>\Savegames\Story' folder.
If that doesn't help, or you re-saved:
Do you have any backups of your My Documents folder, where you may be able to recover a copy of the save(s) before this error started?
Some versions of Windows have a
Shadow Copy feature that can automatically keep older versions of files. You can try right clicking the save folder(s), or the Profile's Story folder, select Properties and (if available) switch to the 'Previous Versions' tab to see if there are any previous copies.
If you email supportdos2@larian.com with your save (or a Dropbox or Google drive link, etc), I can check if there are any corruption related issues that cause errors when loading the save in the developer build of the game. Unfortunately, the checksum value is calculated across all of the save data, so there is no way to identify what changed.
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\PlayerProfiles\<ProfileName>\Savegames\Story' folder.
In Windows Explorer you can zip a file or folder by right clicking on it/them and selecting 'Send To | Compressed (zipped) Folder'.