I have had that a couple of times while i was running around the battle fields....
most of the time, double clicking on my character's portrait, fixed both my main screen that has gone blank as well as the main map...
when that didnt work, pressing "a" for the main map, and exiting again fixed it...
unfortunately i do not know of any other fixes... other than maybe a corrupted dynamic file....
Try deleting the files in the dynamic folders;
'..\Beyond Divinity\Acts\Act1\Dynamic', '..\Beyond Divinity\Acts\Act2\Dynamic', etc.
These folders are used as a cache for the game. Files are created / copied there from the save game folder when you load, and written / copied to those folders when you save a game. The files will be re-created as required the next time you start Beyond Divinity. If there was a corrupt file in those folders, deleting the files should fix the problem.