Is the problem getting to the Dark Forest when you are using the teleporters, or when walking south along the path by the archer's guild to get there in the first place?
For strange problems;
Try deleting all of the files in the '..\Divine Divinity\Run\Dynamic' folder.
This folder is used as a cache for the game. Files are created / copied there from the save game folder when you load, and written / copied from that folder when you save a game. The files will be re-created as required the next time you start Divinity. If there was a corrupt file in that folder, deleting the files should fix the problem.
