AC Black Flag UPlay Bug

Check the .exe in your library. Have similar issues with other uplay_games, that cannot be started over the client (btw. worst client ever, avoid of you can)

You actually bought a Steam (Uplay) AC4 Black Flag.
Owners of Black Flag under Steam receive a Uplay CD-Key in Steam that they then have to use in Uplay. Under Uplay those Steam-Linked copies will always launch using Steam, which it is trying to do right now. Someone registered AC4 Black Flag under Steam and for some reason used their Uplay CD-Key for trade.. that's where you come in. That CD-Key you registered on Uplay is useless unless you own AC4 Black Flag under Steam. The thing is it's also useless for the Steam User who sold that CD-Key to have AC4 in his Steam.. he can't run the game without that CD-Key too.