Where are you in the act? How far in, main game or battlefields?
Do you still have a save at (or near) the end of act 2?
Try deleting the files within the dynamic folders in the install folder (not the folders themselves, though);
'..\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.
After that re-start the game, load the save at the end of act and and try the act transition again, to see if that will avoid whatever the problem was the first time.