Ok, the problem is the incompatible FSharp.Core.dll (4.5 vs. the custom lib in FSharpx.Core)
At the moment we can't depent on FSharpx.Core in FSharpx.TypeProviders. Which is very unfortunate.
I had the same issue witth FSharpx.Http (where I replaced the FSharp.Core.dll)
That's one of the reason why I kept the stuff in the TypeProviders branch. I don't really know how to solve this.
I disabled the JSON provider for the moment, since I want to rebuild it from the XML provider.
I'll try to fix later today.
We need ideas how to structure the projects in order to make it build.