Currently running Windows 10 2009 /20H2
I had this same problem on this game and multiple others. Assassin's Creed 3, Far Cry 3, Saints Row 3 and 4, and others. Had all of them installed on a dedicated, MBR formatted hard drive (not SSD), and none would launch. Previously they would on Windows 10 1909.
Turns out it was RivaTuner Statistics Server, but the "Application detection level" setting was the key. I had it set to High, but turning it to None instantly resolved the problem. All games launch without issue. Uplay is still in online mode too! Though, I believe they still have to be on a MBR formatted drive. Probably setting RivaTuner Statistics Server to not start with Windows, or completely uninstalling it would work as well.
Hopefully this works for others!
Does Ubisoft think this is a joke? How can you allow a critical bug like this to exist for this long... so many posts on this for years - and you have a new game around the corner too. Guess what I won't be buying?
I have a brand new, up to date clean build - Ryzen 5900x cpu, rtx 3080 - no afterburner or any other 3rd party software. Just Steam and the Ubi launcher that doesn't work.
Verified, updated, compatibility mode, offline mode. None of it works.
To those speaking on AA - how do you adjust settings without actually launching a game?
Not cool.
I tried everything already listed in all the forums, Ubisoft's support system was useless and just told me to do what I've already done. Nothing worked and I couldn't launch Black Flag. I ended up reinstalling windows, as a last ditch effort. It worked! You can do it without losing personal files, but back them up, just to be sure.