Another possibility ....
As you are FMQing, check how you are moving the sandwich around under the needle.

If you move the sandwich too fast and far, it could cause the thread to rub against the needle
by making it too tight, and thus the friction could rub the thread,
and then fraying could result from the constant rubbing and added tension.

I am concerned it’s a timing thing. I’d pull the bobbin thread up and try to sew- even slow, feed dogs up or down- no dice. Eventually I put the default foot on, put the feed dogs up, and straight stitch worked. I remember changing to a denim needle which I think solved the problem the first time but not the second. I’m going to pray the polyester thread will make a difference tonite. I am really out of solutions!! Let’s pray. I think my machine is possessed.

did it do it when doing a straight stitch in the denim as opposed to free motion?
that would help determine if it is the free motion action.
That is some seriously fuzzy thread! ;)

It is the fuzziest thread I ever saw! I'm switching to polyester for FMQ. Let's see if that helps!

Ladies and gents...can we revisit this? I changed to polyester thread, was able to sew FMQ for 5 minutes then it wouldn't work anymore. Played with it for awhile, changing everything, rethreading, to no avail. I switched to straight stitch and it worked. Went back to putting feed dogs down and tried FMQ again. Nothing! I am ready to throw either myself or the machine out the window. Any advice or thoughts?

Ok if anyone has had a similar problem, I may have had a breakthrough. I tightened the ruler foot and that helped. I guess I had it too far from the quilt. It doesn't move as easily now because I feel like the foot is pressing too hard, but if I loosen it then it doesn't stitch. So if that helps anyone else who's had this issue, maybe that can help!

I'm not a big proponent of dropping the feed dogs either...

My guess too is of a burr.. somewhere. My reasoning is because poly is strong & stretches. That’s why you got 5 minutes out of it.

Oh. This helped? Well, good.

Glad to hear you have it figured out! Just curious, do you use the ruler foot for FMQ or were you doing ruler quilting? I know some people use the ruler foot for FMQ too so they don't have to switch feet.