There are a couple fixes for this issue.

In the Graphics options, check the 'Show Advanced options' box, then scroll down to the bottom, and just above the FPS Cap slider, enable Lockstep Mode.

If that doesn't help, try the '4GB Patch' to increase the amount of virtual memory that the game (or any 32 bit application) can access in 64 bit versions of Windows.
http://www.ntcore.com/4gb_patch.php

One person reported that neither of those helped, but the game was fine if they disabled VSync and ran the game in XP SP3 compatibility mode (right click on a shortcut, or the divinity2.exe executable in the install folder, select Properties, and switch to the Compatibility tab).
Another with the disk version of D2:ED (so no Lockstep Mode option) said it helped to enable 'Single-Core Optimized'.


One person reported that setting the game to run in a window fixed the problem. To do so, edit the file below in a text editor and change the one to a zero in Fullscreen="1".
C:\Users\<UserName>\AppData\Local\Divinity 2\Profile\graphicoptions.xml

You may need to set Windows Explorer to show hidden files and folders:
http://www.howtogeek.com/howto/windows-vista/show-hidden-files-and-folders-in-windows-vista/
(covers Vista through Win 10)