Just to put your mind at rest.
The problem rarely manifests itself before 12000 miles and for some reason the 2002 to 2003 foxeyes seemed to be the worst affected as were many other Yamaha 600s from 2002 to 2006 which used the same type of 9mm nut.
The new nut and washer comes in kit format which includes both and part number is 90891-10124
The problem is cause by the tolerance allowed by Yamaha meant the shaft could be machined too small and the nut could vibrate and the threads fret causing the threads to wear away, losing all thread and then falling off.
Yamaha produced an uprated nut 12mm thick instead of 9mm and recommended a new torque on the thicker bolt of 90Nm.
Do not torque the 9mm nut to 90Nm as you are likely to strip the threads in the nut, there is insufficient thread in the nut to take the 90Nm safely.
This solution works as long as the threads had no damage.
Unfortunately the first time it happened on mine the sprocket fell on to the threads slightly flatting them, exacerbating the problem. After much communication, (I am being nice here) Yamaha finally relented after 72000 mile when the threads of the shaft were almost completely gone to supply the all the parts to repair it (all the gaskets, shaft, bearings, seals and other items) at half price, because they were supposed to have repaired it at 15000 miles when it first happened.
I documented my fix for the problem and uploaded it to the download section. This fix has over 150000 miles on it without problems. I just replaced the Chain and sprockets recently and the fix seems to be effective.