During the fight with the necromancer witch in surrey crypt, if a skeleton summon or the witch is defeated, the game crashes. This happens on the lowest graphic setting in windowed mode, and with every possible graphic setting turned off. Does anyone have an idea on how to fix this?

This is a known issue, and there is a patch in the process of being finalised for testing.

You may be able to work around this by CCing the summoner or killing them quickly enough.

If you are playing on PC, email supportdos2@larian.com with your save there (or a Dropbox or Google drive link, etc) and I can probably end the combat in the developer build of the game. If you are using any non-Larian mods, also include a list of those enabled and/or the modsettings.lsx file from your profile folder.
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' (and similarly in OSX/Finder).

I just kill her while she's petrified.

Alright, thanks! I will send you an email with the save file then. I don't use any mods at the moment, so I don't think that's an issue