Right. So, I did another reinstall of all FSDT products, including the addon manager with antivirus completly disabled, and only the one exclusion for the manager itself, but got the same error. (see the screenshot).
For some reason I'm not even asked for conformation like "Do you wanto to use Add-On Manager" after a complete reinstall. It just reloads the scenery database itself altohugh, all addons, like FSDT Exclude, KIAH and KLAX airport seem to be installad in the scenery libary itself, as well as the addons tab (and yes GSX and the addon manager showing up in the menue bar at the top

)