__Not working on Nvidia 390.x drivers __
I tried your instructions sudo nvidia-xconfig... to set cool bits to 28 and then rebooting. I think it auto rebooted once again.
When I then tried setting the FanSpeed... still got the same error (Unknown error)
My guess, when it auto rebooted the second time (not sure why/how that happened), it recreated the xorg.conf file and overwrote the coolbits 28 that I had added. I know this for sure coz after using the CLI to add the coolbits I checked the updated .conf file.
Looks like Nvidia changed/disabled coolbits updates to .conf in their newer drivers