Is this only with one particular gear (say, 3rd) or does it happen (sporadically) over most of them? If it's one gear it may be something internal in the gearbox but if it happens over a number of them I'd start by checking the shift mechanism at the other end of the shaft the gear lever bolts to.
There are various springs and screws there that will give the vagueness you describe if they come loose / break. I don't know the YBR specifically but it's commonly found behind the clutch basket so the side cover will have to come off to have a look. At least that's quick to do.
I've got a similar problem with an old 650 twin Yamaha at the moment but it's only with 5th gear so I suspect the whole thing will have to come apart. Something for the summer though, it's just too cold now.
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