I have done all the checking short of reinstalling the C++ 2005 libraries
Well, the missing/corrupted libraries are one of the most likely cause (excluding a bugged antivirus), so you should have tried that. And no, fixing the libraries doesn't require reinstalling FSX at all, in the worse case you will need an FSX "Repair" option, not a reinstall.
But this is old information. The current version of the Addon Manager doesn't require the VC++ 2005 runtimes, but the VC++ 2013 ones, which might be or not already installed in your system but, since this is downloaded automatically by any of the installers, if you run an OLD version of our installers, and you don't have the VC++ 2013 runtimes installed in your system, it won't work so, you can do one of the following:
- Download and install the Stand-Alone Addon Manager for FSX, which now contains and install the VC++ 2013 runtimes
OR
- Download and install the VC++ 2013 runtimes from Microsoft
OR
- Download and install the latest version of the full installer of the FSDT product you are trying to reinstall, because they are all being updated to install the VC++ 2013 runtimes too. This might be a bit overkill, but it will work too.