Other fans have also low speed warnings/errors.

Check for errors/warnings related to the +24V of the SMPS.

Blocked fan.

Check if the fan can turn freely.

Fan disconnected.

Check fan wire connections (reference 1 image)

Damaged wire.

Check if the wire of the fan is not damaged. Repair if possible, otherwise replace with new one.

Fan end of live.

Replace the fan. See service procedure "Replacement of the Card Cage fans".

Malfunction CCB.

Replace the CCB, see service procedure "Replacement of the Cinema Controller".