You are not alone! I am getting this error, also.
I have turned off just about every background process I can think of (without crashing Windows 10, of course) including FSX Steam Edition, disabled all FSX Steam Add-ons, Windows Firewall, Sophos Antivirus and HitmanPro.Alert service so very few processes are, still, shown as "running".
I have also added the folders "C:\Users\MyName\AppData\Roaming\Microsoft\FSX", "E:\Program Files (x86)\Steam\steamapps\common\FSX" and "E:\Program Files (x86)\Addon Manager\" to my Sophos Antivirus Folder Exclusions settings.
Yet, when I download and run the latest version (August 26, 2017) of "setup_addonmanagerX.exe" I receive the pop-ups: "Runtime Error (at 144:1485): Out Of Range." quickly followed by "Error 3 running E:\Program Files (x86)\Addon Manager\Couatl_Updater2.exe which means The system cannot find the path specified."
I performed a manual search in "RegEdit" and nothing (meaning, no path) shows up, anywhere, in the registry under "Couatl_Updater2.exe" so (crap!) frustration, again!