The bike was still under manufacturer warranty so I took it to the dealer earlier this week. Went to pick it up again this morning. The verdict: 1 frictionplate was totally gone , the others seemed fine. Plenty of metal particles in the oil and the oil filter .
A whole new clutch was installed under warranty. In theory clutch plates are wear parts, but the dealer and Mr Yamaha agreed that at only 16000km and with the other plates still looking good it was probably a manufacturing fault.

So I'm happily toodling along on my Ten again...

Will just do an oil change in a couple 100km's just to make sure all metal particles are really gone.