It's not really possible you could have two Addon Manager installed at the same time because, when you install it, the new version replaces the old one, even in the Windows Control Panel.
And, that error means that you still have something not updated, most likely you have an old version of Couatl.exe, which is not normally possible, since that one is downloaded online too.
However, it's possible your antivirus has blocked the download of the current version. Check the fsdreamteam\Couatl\Couat.exe it's version 2.0.0.2090.