I have one one 66-4 that will break Gutermann thread about ever three inches. Coats & Clarks and Mettler work great. My other 66-4 hates C&C machine quilting thread but will sew with most anything else.
I have another one that couldn't care less what thread I put through it. It 's a 66-18. I guess you can say that machines are like people, they're individuals.
I've read that Mettler thread is discontinued having been bought out by someone. So perhaps the machine is just telling you it want's a diet of something that can be had down the road.
Joe