Yes, it might be a bit confusing. x264_opencl is built with opencl support but not enabled by default. The reason why an older x264.exe is kept there is for encoding on systems without opencl runtime.
Error message file should be created in the folder of MediaCoder.
x264 Lookahead with OpenCL and Mediacoder
Re: x264 Lookahead with OpenCL and Mediacoder
When things work together, things work.