Receiving the same error when attempting to save. I was originally wondering if it is because I my 'my documents' is located on a NAS drive, but less sure now.
However, one thing to note, Larian, in my 'my documents' directory I see two paths for save games:
\\NETGEARNAS\data\Files\Documents\Documents\Larian Studios\Divinity Original Sin\Savegames
&
\\NETGEARNAS\data\Files\Documents\Documents\Larian Studios\Divinity Original Sin\NETGEARNAS\data\Files\Documents\Documents\Larian Studios\Divinity Original Sin\Savegames
The UNC name is also mapped to a drive letter.