It's not possible this error could have anything to do with GSX. None of our software installs, use, is depending from or has any relationship with, the .NET framework, not even our installers. Our software modules are written entirely in C++ native un-managed code, with no relationship whatsoever with .NET, any version of it, the one and only system libraries we rely on, are the VC++ runtime libraries, which are also used by FSX itself.
Instead, there are *lots* of other FSX 3rd party add-ons that use .NET framework so, that error is coming from one of them. The .NET exceptions usually have a dialog that allows you to see error details, so it should be possible to see which program has caused it.