Did you use the same strategy in both fights? If it is the actual death causing the crash, maybe killing the void dragon in a different manner will help.
Try lowering the resolution and graphics settings, and/or switching the game to a windowed mode.
For the Steam version, did you try verifying local files? In the library, right click on Divinity Original Sin and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Cache...' button.
For the GOG version, if you kept the installer (and patch(es), if you downloaded one of the older full installers), you could uninstall, reboot, make sure there is nothing left in the install folder, shut down all non-essential programs (especially anti-virus) and re-install.
Please email supportdos@larian.com, with a brief description and your latest save before the fight. Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin\PlayerProfiles\<ProfileName>\Savegames' folder. In Windows Explorer you can zip a file or folder by right clicking on it/them and selecting 'Send To | Compressed (zipper) Folder'.
