That's not a problem of the sceneries, your issue is a problem of the MS Visual C++ runtimes.
Of course, all our sceneries requires the MS VC++ runtimes to be working, both for the Addon Manager and the Couatl engine, but that's only about the relationship with this .dll and the sceneries, they all require the MS VC++ to be installed properly.
You are not seeing the problem with other sceneries, since they are regular .BGL file with no special features that requires C++ runtimes so, they are not affected by them being installed properly or not.
A while ago there was a known bug in the 2008 version, which caused a crash in MSVCR80.dll which is instead part of the 2005 version, but installing the 2008 runtime together with 2005, caused a problem with that .dll
FSX itself comes with and requires the 2005 runtimes, and our sceneries also automatically install the 2005 version in any case, but I think some PMDG airplanes install the 2008 version, and any other Windows program you installed in the meantime could have done this, which is why you said yourself that FSDT sceneries used to work fine.
I'm not sure if this bug has been fixed or not in subsequent version of the runtimes, I guess it should have, since it quite serious but, if some of your C++ runtimes files are not updated, it might still be in.
You said you have already uninstalled and reinstalled everything. .NET wasn't required, so the video drivers.
Try this:
- Uninstall ALL the MS VC++ runtimes (all versions 2005 and 2008, both their x86 and x64 versions)
- DO NOT reinstall them
- If you installed the Stand-Alone Addon Manager, Uninstall it and reply YES to the questions "Do you want to remove the Addon Manager?" and "Do you want to remove the Couatl engine?"
- Reboot Windows. Please, do it.
- Reinstall the Stand-Alone Addon Manager and try FSX again.