I had the same problem last month... with the same vintage bike!
This is due to electrical connectors in the neiman... the only way is to change the keylock.
Of course that's expensive (270€), of course that's need to have 2 keys (because they only change the neiman, and not all 7 keyslock!).
I m really angry against Yamaha, but I love too much this bike...
(I m apologies for my spelling errors... I m from France )