Optiplex 760 Rear Fan Failure

I am reworking a Dell Optiplex 760 and got it to run Windows 10.
When booting, the post screen showed a message "rear fan failure" and to press F1 to continue.
First I checked all fans, the message is related to the fan mounted below the 3,5" HDD.

But all fans were turning fine.

The problem I found was a contact problem caused by the fan cable plug on the motherboard.

There are 5 pins soldered into the board, a plastic plug holder is just pushed onto the pins, with no
fixation. The fan cable side of the plug is then to be pushed into the plastic part and clips in.

Someone seems to have tried to pull out the fan cable plug and instead of getting the plug out of it's plastic holder, pulled out the plastic holder itself, about 3mm off the board. That was ok for giving the fan power but disconnected the sensor lines.
I pushed in the holder with the plug clipped in until it sat flat on the bord, and all worked fine.

So, if you ever see this failure, check if the plug holder sits flat on the board without a gap showing the pins. If it does not, just push it in and restart the machine.