Did you have Bless cast, or eaten/drank anything that boosts strength? If that effect wore off, it could explain becoming encumbered.
weight bug and disappearing chestsTry saving the game after having dropped heavy items, then delete 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.
After this re-boot, shut down all non essential programs and try loading the game again.