Assassin's Creed 3 save files are linked to specific Ubisoft Account IDs. If you paste a save file from another player, the game may read it as corrupted. To fix this, you must use an AC3 Save Tool/Convertor (available on community forums) to re-sign the save file with your unique Uplay Account ID.
Whether you lost your original data, encountered a progression bug, or simply want to replay the best part of the campaign, using a Sequence 5 save file is the perfect solution. Why Start at Sequence 5?
The Davenport Homestead, right before or just after meeting Achilles.