We haven't updated Couatl in a while. The last update updated *only* the Addon Manager but, if you have it running, that cannot possibly the problem. Couatl.exe hasn't been touched.
So, the only possible reason why you see the Addon Manager but not Couatl.exe, is an update of your antivirus. As explained in all threads about the antivirus, you must add the Addon Manager folder to the antivirus exclusion list and, if you have a firewall, be sure you give permission to the FSDT Live Update to access the internet.
No, there's no need to disable the antivirus, but you MUST exclude the Addon Manager folder and be sure the Live Update can download in that folder, because if the antivirus/firewall is blocking some files and allowing others, it will result in a mismatch of files belonging to old and new versions, which will very likely cause the program failing to start.
If you had a Windows update recently, try a Live Update after a Windows restart.