Are you shutting down all non-essential programs before starting the game? Anti-virus and firewalls are the most likely to cause conflicts, but other programs can, as well. A couple people have reported crashing due to Razer Synapse (bundled with Razrer keyboards, mice and headphones).
Try exiting out of the Steam client, and starting the game directly from the '..\SteamApps\common\Divinity Original Sin 2\bin\EoCApp.exe' program file, by right clicking and running as administrator.
Try lowering the graphics settings and resolution, or switching to Windowed or Fake Fullscreen display mode, to see if that makes a difference, if rebooting or other suggestions let you get that far, but you still get crashing during gameplay.
Do you have Vsync enabled, or the frame rate cap set in the options? Try enabling, lowering or disabling those options.
Check for updated graphics drivers and Windows updates in general.
If you check the Event Viewer, does that give an error code or 'Faulting Module' file name that might help identify the cause of the crash?
- click Start (or WinKey-R or WinKey-W in Win 8), then type "event viewer" into the search box. in Windows 10, 'event' should bring up 'View event logs'.
- after starting the Event Viewer, expand 'Windows Logs' in the left column and select 'Application'
- in the center column, look for a recent error (maybe sort by Date and Time) for the game
- check the information under the 'General' tab below the list of events, starting with "Faulting application name..."
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).
Next, browse to the '..\Documents\Larian Studios' folder and rename the 'Divinity Original Sin 2' 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 cloud support enabled, Steam would just download the cloud copy of your existing profile.
After that, extract the replacement folder from the download below into your '..\Documents\Larian Studios' folder, and see if that will let you start the game and get to the main menu.
https://www.dropbox.com/s/kjvq79igruaeu4h/DOS2_doc_20165341.zip?dl=0The graphicSettings.lsx file is set to 1280x720 Windowed mode, Very Low quality preset, which you can change in the options (manually, or hit autodetect) if this gets avoids the crashing.
If that doesn't help, you can delete the downloaded My Documents D:OS 2 folder and rename the original back again.