H264 encode fails on Windows 10

I'm using a windows build of ffmpeg to encode with, and it works well on Windows 7, but periodically fails on Windows 10.

The symptoms are dependent on the Intel graphics driver version.
Intel(R) HD Graphics 4600 20.19.15.4352 Active
Intel(R) HD Graphics 4600 20.19.15.4463 Active -- returns error
You'll either see ffmpeg appear to lock up, or return an error "encoder failed".

My debugging shows me that it get's stuck in a loop calling MFXVideoENCODE_EncodeFrameAsync()
which returns MFX_WRN_DEVICE_BUSY continuously with the older driver.
The newer driver returns MFX_ERR_DEVICE_FAILED.

This occurs on Windows 10, but not Windows 7. it happens about 40% - 50% of the time at the start of an encode.

It seems to occur more often at higher resolutions like 1920x1080.

Could this be caused by incorrect init parameters?

Any insight into why this might happen would be greatly appreciated.

I'm attaching mfx tracer logs - one successful, one failed.