Maybe it's not so important but you can try. I have one rig and it's working very well and on another one I had some problems.
I've tried with windows 10 1803, windows 10 1709 16299.192/402 and several drivers for nvidia.
I said..ok, let try to install 1709 untouched, win 10 1709 16299.19 and had the same issue - "unable to get fan speed - unknown error (999)" and every time was on PCI x16 and tried to change the GPU's between them but always the same ... first GPU.
With that windows 10 1709 16299.19 I said..let try to update, and now it's updated to 16299.431 (the same with the other rig that it's working without 999 error), the driver updated itself to 388.13 (previous was 387.92) and it's working for the moment. I'm still testing.

I will come back with feedback after another 1-2 days

Regards