It may be the needle you're using with that particular kind of thread. Happens to me with my Elna now and then. It doesn't seem to like 90/14 quilting needles with poly thread. Prefers cotton!

This Bernina has sewn fine with all threads up to now so there has to be a reason why it won't now. It is at the shop. I took both spools of thread and wrote on the ticket the details of why I brought it in.

Hope you get it figured out and it's not too difficult or expensive a fix. Let us know the result. You've got my curiosity piqued.

Rob