Mmmm windows 8.1 does not seem to have a F1 key on the virtual keyboard.
I tried another trainer for max payne and this one is working fine.
When i start AC black flag without trainer and exit no problems. When i try it with the trainer and after hitting F1 the computer completely freezes when i try to exit the game.
I'm sure you've already checked it, but this has happened to me before if my trainer is out of date.
If you've purchased any new DLCs or anything recently, it might be a good idea to download the latest version of the trainer again and see if that helps.
Is there anything I can do for the tail/follow missions? I am not very good at them...
You can try hiring some prostitutes if there are some, pretty easy as they're hiding you all the time. Apart of that I have no idea.
Game crashes the moment I activate it.
Playing the latest version through Steam/Uplay, and tried running in Admin mode.
Even if you activate the trainer at the main menu?