There is a spacer that fit inside the r/h seal between the bearing and the axle....it usually doesn't fall out that easily so it might still be there.
have a look at the parts catalouge here:
http://www.ducati.com/bikes/service/...delName=200 4 If the wheel is re-fitted without the spacer the discs will probably foul the calipers. Worth rechecking it's there!
Good luck