your not going to like this!!!!! inside the gearbox is a small bolt that locks the gear shaft to the shifter,, its come loos? all you can do is strip the gearbox and nip it back up again,,,you can test this yourself,,,
try lifting the gear leaver up and down,,,if there is a lot of lift in it,,its that darn bolt loos!,,,go on the net and look at sight,
www.maxbmwmotorcycles.com you will find the gearbox on this sight
and that bolt! sorry???