I have had this problem on my 510 and my 550.
In both cases the problem was caused by dirt and rust Build up on the transmission input shaft spines, and on
The driving lugs on the steel intermediat plate which is between the friction discs
All that it took to fix was a good clean up job.
Can be bit tricky to get the outer clutch disc splines to Line up when reassembling, but overall not a difficult fix
The clutch must be taken off in pieces, as the final drive housing prevents it from being taken off as unit.
About a 2 hour job .