ML10 1611-Fan 1 Failure Fan Solution Not Sufficient False positive ?

[tamask]

Half a year ago I set up and run ML10 server. It worked properly. After 3 months it lost electricity. On power restore it failed to boot with this error:

1611-Fan 1 Failure
Fan Solution Not Sufficient
Critical Failure Detected - System Shutting Down in 5 seconds

I left it to sleep for two days and try to boot again - it booted and worked properly for another 3 months.

Now after another electricity lost it refuses to boot again with the above fan failure.

Fan-1 is system fan, Fan-2 is CPU fan. Both are working on full speed. Both has 6pin inline connector. I measured signals on fan 1 connector ... red wire +12V, black GND, blue PWM - voltmeter shows +3,5V;  yellow "OK" is +3,5V when it starts, but it falls to 0,15V in a second, therefore I think Fan is OK.

Ambient temperature is 20°C, highest temperature (40°C) is measured on CPU, all OK, so I do not understand, why is "fan solution insufficient" ?  Is there any way how to disable Fan 1 checking in BIOS ?

Any tips appreciated ! TIA !