I'd like to share a possible solution.

In my heavily concurrent application, I had the similar problems, though I had two possible error codes: ERROR_TRANSFORM_NOT_SUPPORTED or ERROR_INVALID_HANDLE, both appearing in an apparent in-deterministic manner, indicating a multithreading issue. I figured out that wglMakeCurrent ist *not* reentrant (at least when called on the same device context HDC, i didn't test the scenario when both, HDC and HGLRC, point to a different handle). By creating a spin-lock around wglMakeCurrent the problem disappeared for me.