Launcher crashing / game will not start from the client

Try exiting out of the Steam client, or Galaxy for the GOG version, and starting the launcher from the '..\SteamApps\common\Divinity Original Sin 2\bin\SupportTool.exe' executable.

If the launcher doesn't start from the executable, try deleting the config.xml file in the '..\Divinity Original Sin 2\bin' folder, as well as 'C:\Users\<UserName>\AppData\Local\LarianLauncher' (copy and paste %LocalAppData%\LarianLauncher into Windows Explorer to quickly open that folder).
After that, try verifying local files: in the Steam library, right click on the game and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Files...' button.
With the GOG version, in the (optional) Galaxy client, select the game, then the settings icon at the top right (beside the Play button) and under Manage Installation select 'Verify / Repair'.

If that doesn't help, try reinstalling .Net Framework 4.
C:\Program Files (x86)\Steam\SteamApps\common\Steamworks Shared\_CommonRedist\DotNet
C:\Program Files (x86)\GalaxyClient\Dependencies\__redist
or https://www.microsoft.com/net/download/dotnet-framework-runtime

If that doesn't at least get the launcher starting from the executable, there is also a Microsoft .NET Framework Repair Tool, or if that doesn't help, Cleanup Tool.

Try starting the game directly from the '..\SteamApps\common\Divinity Original Sin 2\DefEd\bin\EoCApp.exe' program file, by right clicking and running as administrator (or '..\Divinity Original Sin 2\Classic\EoCApp.exe' for the original release).

If the game gives a VC related dll error when starting from the executable, try installing the 2017 Visual C++ redistributable (64bit), now included with the 2019 release.
C:\Program Files (x86)\Steam\SteamApps\common\Steamworks Shared\_CommonRedist\
C:\Program Files (x86)\GalaxyClient\Dependencies\__redist
or https://support.microsoft.com/en-ca/help/2977003/the-latest-supported-visual-c-downloads

If that doesn't help, in the Control Panel, Programs and Features, try right clicking the MS VC++ 2015-2019 redistributable entries (32 bit and 64) and select Uninstall and then Repair. If you still get the error after that, right click again and uninstall, reboot and reinstall, then test the game from the executable.

If the game starts from the executable, but not through Steam, 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). Also in the game Properties, try disabling the Steam overlay in the General tab.

If that doesn't help, please contact support 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 'bin' folder for the launcher and/or game, depending on which is crashing, for the gold.log file, and if there are any 'CrashDump' files being created, zip a few together to include.