I turned off my antivirus software and restarted your newest "Setup_addonmanagerX" again. I left the antivirus program off this time and started FSX. Everything works again! The uncanny thing that bothers me is, I don't understand why this situation is resolved and why it happened!
A likely reason is: the current version of the Addon Manager .dll (which is downloaded automatically, even by older installers), requires the VC++ 2013 runtimes from Microsoft to be installed.
If you don't have them on your system already, it won't work, but if you download any current FSDT installer, either a product like GSX or XPOI or a scenery, or the Stand-Alone Addon Manager, the installer comes with the VC++ 2013 runtimes and installs them automatically, that's what was fixed your issue, most likely.
