After launching the game from uplay or steam two error messages will pop-up: "Unable to detect Uplay installation. Please make sure uplay is installed" and "Untusted system file (C:\Windows\System32\uxtheme.dll)"
Can you please uninstall Uplay, and install it from this link
With regards to the .dll error you are getting, I would advise running a full virus scan of your system.
Please let us know if you are still having issues after this
Did both things you asked. Still same issue. Other ubisoft games launch without a problem. Also tried deleting the game and installing it in another directory. Issue still present.
Do you have the game installed to the default Steam installation directory?
Is it installed on an internal or external drive?
Also, do you have Steam and Uplay installed on the same drive?