Error 200 is an issue with a cache file when loading a level.
Do you have a save before saving the White Witch that you can revert back to, and redo that section?
Try browsing to the '..\Documents\Larian Studios\Divinity Original Sin Enhanced Edition\LevelCache' folder and delete the contents. A corrupt file there can cause problems when saving or loading, though if earlier saves are still ok, the issue is most likely a problem with the latest save.
If you are playing on honour mode difficulty, or otherwise don't have any recent saves:
Do you have any backups of your Documents folder, where you may be able to recover any saves?
Some versions of Windows have a Shadow Copy
feature that can automatically keep older versions of files. You can try right clicking the Profile's Story folder, or individual save folders, if applicable, select Properties and (if available) switch to the 'Previous Versions' tab to see if there are any previous copies.
If that doesn't help, you can email email@example.com with your save (or a Dropbox or Google drive link, etc).
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin Enhanced Edition\PlayerProfiles\<ProfileName>\Savegames_patch' folder.
In Windows Explorer you can zip a file or folder by right clicking on it/them and selecting 'Send To | Compressed (zipped) Folder' (and similarly in OSX/Finder).