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CPU Fan Speed Error on boot

Level 7
Out of the blue it seems I am getting a BIOS UEFI error on boot.

Getting an error (picture 1) stating
CPU Fan speed error detected. Ensure to install the CPU fan properly onto the CPU_FAN header or adjust/disable the Fan Speed Low Limit option in UEFI.
Press F1 ro Run SETUP

So I run setup (picture 2)
The first thing I check is CPU temperature which seems normal at 36C. If my water cooling had failed it might explain the high fan speed (Ive experienced water cooling failure on the same machine in the past)
When I open the QFAN control you can see that my fan speed is set to maximum. (picture 3)So I adjust to silent or Standard and exit. One thing I notice is that when I make a change, the 'Apply' button doesn't become highlited. And what is the difference between PWM and DC? What should I use?

Then Save&Exit from the main screen to boot Ubuntu.

But the error persists(my CPU fan stays at maximum).

Any suggestions what I should try next?
Running Ubuntu 16.04, Asus X99-2 mobo
Attached Thumbnails649066490764908

Level 7
problem solved by switching from CPU OPT fan header to CPU fan.

But puzzled why it was working fine for several months with CPU OPT?