Both as a Crusader pursuing Metrodora Thelema for recruitment, and as Assassin trying to access the Martyr after getting Uther's Tarot, the game refuses to load the mission, instead reverting to a frozen loading screen that fades to white and offers windows to close the game. The options to end the task/close the game simply result in the frozen game window staying frozen. Restarting windows is the only way to successfully close the program, which refuses to do anything at all until restart occurs.
The effect seems contagious. After freezing the game on the loading screen, that character cannot successfully load any other missions until such a time as another character has successfully loaded a mission. Then, the freezing resets, I can play side stories again, but the main mission remains bugged and I cannot make any progress. Returning to the main mission after playing side missions results in the game, again, crashing.
For confirmation, the freezing occurs at the end of the loading screen, when the loading bar is full and the mission should be about to start.