It looks like the joint where the headstock section is scarfed to the main part of the neck. I'm very surprised to see that joint fail. Great basses, btw.
Yamaha NTX3 player here. Nicely made hybrid that plays very well, with a slightly thinner body that a regular classical. The pickup system is very good, with subtle but useful EQ.
I have one of these. I think the pickups in these are P-style splits inside. I just tested this theory by using an Allen key on top of the pickups. Definitely more pull on one long edge than the other on the treble side, and reversed on the adjacent pair of bass strings.
hope this helps.
There is also a mod for the Sub series original preamps, which were very hot. It doesn’t look very difficult, and I have a schematic if you’re interested. I haven’t done it but it’s because I’m heart lazy.
As suggested above, try this email address: [email protected]
The guy who replied to my particular enquiry was Huw Lloyd-Williams. Despite the email address he is in the UK.
Roland have a list of certified repairers, which is what Huw might direct you to: https://www.google.com/maps/d/viewer?hl=en&mid=1ufxdlNwWP6MtTncc3gCTcrqIUXA&ll=55.25912902791857%2C-0.2807196689598612&z=6