That error doesn't happen here. Try to download and install the full GSX installer, instead of just the Addon Manager.
If that doesn't work, try to remove the following folder:
C:\ProgramData\Microsoft\FSX\SceneryIndexes
(note that C:\ProgramData is hidden, so you'll have to enable hidden folders view in Explorer)
This will force both FSX and GSX to rebuild their caches. When GSX it's done, exit from FSX and enter it again, and see if the problem gets fixed.