First solution: Make a shortcut of Assassin's Creed III on your desktop. Than right click on the shortcut and click on Run As Administrator.
If that does not work...
Second solution: Delete the Assassin's Creed III saved game folder (54). Start the game. Create a new game using the name Saletor. Play the game until you have completed the Opera House missions. Once you have reached that point exit the game. Copy and replace all files from folder (54) with my saved game files. Don' copy and replace the folders just the files within the folder.
Please let me know if any of these solutions helped. Good luck!