I don't think that installing sceneries again would made any difference, their installer downloads online the same version of the Addon Manager that GSX uses.
There must be a problem with some system library instead that has gone missing, so you might have to repair FSX instead.
But, before doing that, try to remove the FSX.CFG at %APPDATA%\Microsoft\FSX and let FSX rebuild a new default one from scratch, sometimes it helps.