Apparently, it’s a rare case, and seems to occur if you try to load your save file while downloading the new DLC, “Hidden Secrets”. As per usual, they are working on a fix for the bug.
“Do not load your saved AC3 game while the DLC is downloading in the background. This appears to be the key trigger for this bug. You can do anything else you want to do, just don’t load up your saved game and play it while the DLC is downloading.”
Last thing you want is to prepare for new content to realize you have to do the core content all over again. You have been informed.
Article from Gamersyndrome.com