User blog comment:One True Slash/Assassin's Creed Unity Resolution Lock/@comment-3427635-20161213013838

Hmmm... That's a weird issue. Try right-clicking on ACU.exe -> Properties -> Compatibility -- there might be some resolution options set in it for you.

Also try checking "%APPDATA%" and "%APPDATA%\..\Local" in Windows Explorer for Uplay or Ubisoft folders. See if there's a leftover cache after you uninstall.