It might be the error that some get when the fan doesnt spin up fast enough. I've heard of it happening a fair number of times with ASUS motherboards. If this was the case then your motherboard was getting angry that the fan didn't reach a certain preset rpm value when starting up, so it thinks the fan is having issues (which, if it was, would mean it wasnt cooling your cpu properly, which would be why the alert is there). Generally it's preset around 600 rpm, but sometimes it might start up between 400-500 rpm and set off the alert. You can go into the BIOS settings and change the minimum value for the alert message to something lower. Maybe monitor temperatures in BIOS for a few minutes to make sure everything is going properly. Basically, it could potentially be an incorrect message. If it lets you get into BIOS and you see the fan holding steady at or rising to a certain temperature then it is probably fine