.) send some reports to larian studios
The crash report program submissions go directly to the programmers, so they can help with future updates, but there is no feedback mechanism.
If applicable, disable Steam cloud support either globally (in the client click on the Steam menu and select Settings, and then Cloud) or just for this game (in the library right click Divinity: Original Sin 2 and select Properties, then switch to the Updates tab and check the Steam Cloud section).
Alternately, exit out of the Steam client and just start the game directly from the executable when required.
Next, try browsing to the '..\Documents\Larian Studios' folder and rename the 'Divinity Original Sin 2 Definitive Edition' subfolder.
This folder contains the saved games, configuration files and a level cache folder. Deleting or renaming it will get the game to recreate it on startup; playing the game from a different Windows user account would effectively do the same thing. With Steam / Galaxy running and cloud support enabled, the client would just download the cloud copy of your existing profile.
After that, extract the replacement folder from
this download into your '..\Documents\Larian Studios' folder, and see if that will let you start the game properly.
The graphicSettings.lsx file is set to 1280x720 Windowed mode, Very Low quality preset and low audio quality, which you can change in the options (manually, or hit autodetect) if this gets the game to start.
If that helps, create a new profile, exit and copy a couple saves from the renamed folder into the newly created profile's ..\Savegames\Story folder (if Steam synced previous saves, or you copied them over from the old computer). If that continues to let you start the game (and load the saves), move the rest of the saves over.
If the game still crashes, delete the replacement My Documents D:OS 2 DE folder and rename the original back again.
If that doesn't help, please email supportdos2@larian.com with a dxdiag report (WinKey-R, type in dxdiag and hit enter, then when it finishes loading click on the 'Save All Information...' button and save the report somewhere handy). Also check the '..\Divinity Original Sin 2\DefEd\bin' folder for the gold.log file, and if there are any 'CrashDump' files being created, zip a few together to include.