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.
If that doesn't help, check out the install verification method described in
[i]Troubleshooting and Manual Install Procedure[/i]. Maybe the file containing the death animation / sound effect for dragon riders is corrupt? Try to Polymorph one (after Curse if their resistance is high) if you have the skill and see if this makes a difference. If all else fails, you could just avoid them; there are not that many in wastelands.