A connected USB device was "my" problem
Hello,
my singleplayer also didn't want to start (multiplayer had no problem, which made this even more confusing).
My "solution" (if it can be called that) was to unplug my keyboard (Wooting Two) and the game just ran without issues.
As soon as I tried to plug it back in, the game crashed. (Isn't really a problem for me since I always use a controller to play AC, but I'd have liked to change my profile name from "Abstergo" to something else)
To be fair, my keyboard has a feature called anolog input, which let's you use the keys like the triggers on gamepads. To enable this funtionality the keyboard acts as a controller. Certainly a unique thing, but I thought I'd mention it here to let you know.
Maybe check what you have plugged into your pc, I can prevent this game from starting.