If you create a new profile and start a new game, can you save and reload? What about in combat?
Does it make a difference if you return to the main menu first, then try loading the save?
Are you shutting down all non-essential programs before starting the game, especially anti-virus and any backup programs that may be monitoring the My Documents folder?
For example, a few people have had BitDefender block the game from being able to create files and folders, which has caused issues when saving or loading, though usually errors rather than crashing / rebooting. In those cases, including both the game executables and Larian Studios My Documents folder in BitDefender's trusted app and directory sections would fix the problem (Advance Threat Defense, Ransomware Protection / Safe Files, Application Access).
Try exiting out of the Steam client, if applicable, and start the game directly from the '..\SteamApps\common\Divinity Original Sin 2\DefEd\bin\EoCApp.exe' program file, by right clicking and running as administrator.
Try lowering the graphics settings and resolution, and switch to Windowed display mode, to see if that will make a difference.
If a new game in a new profile also causes a reboot when reloading:
Try creating a new Windows administrator user account (preferably with the My Documents folder on a different drive), switch to that account and try starting the game from there, directly from the executable.
Try doing a clean boot and then test the game. Click Start, or hit WinKey-R, type in msconfig and hit enter; in the General tab, click Selective Startup, uncheck Load startup items (if required) and leave Load system services and Use original boot configuration options checked. Next, click on the Services tab, check the box to Hide all Microsoft services, then click the Disable All button (maybe make a note of which are currently enabled/disabled), then click OK and reboot the computer.
Run msconfig again to switch back to the normal boot configuration.
If a new game is fine, please email email@example.com with your latest save (or a Dropbox or Google drive link, etc) and if older saves are ok, the most recent save that you can load and re-load, and the oldest that you can not.
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'.