"Just a quick heads up, we're able to reproduce this issue at the studio and are working on a fix/workaround. From what we can tell, this problem seems to be rare and should generally only affect a very small number of players. That said, we're taking the speedy and complete resolution of this very seriously."
Barring a complete fix, Ubisoft offered a tip to avoid the glitch:
"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."
Hidden Secrets launched for Assassin's Creed 3 Season Pass holders on December 4 and drops for the public on December 11.