I've had a sort of similar issue with wwDotNetBridge where something else on the machine was loading an earlier version of the CLR (v2.0 in my case) in the same process before my wwDotNetBridge code (which required CLR v4.x). At that point wwDotNetBridge would fail on calls to methods that didn't exist in the earlier .NET.
It's supposedly possible to make wwDotNetBridge load a particular CLR version explicitly but I could never get it to work so I ended up rewriting that part as a regular .NET DLL with the relevant methods COM-visible, and using it in VFP that way.
Maybe Rick Strahl would be of more help on the West Wind forums.
Basically