When a regular sewing machine has this type of problem, it is usually the thread in the bobbin. A small piece of thread can cause a big thread problem. I Googled it and found this answer in a Ricoma website, but I suspect it will apply to your machine too.
https://ricoma.zendesk.com/hc/en-us/...0direction.%29
Brother has this to say:
https://www.brother-usa.com/blogs/st...h-these-4-tips
Good luck! When this happens, it frustrates all of us badly.
fyi....this was my search parameter: upper thead causing a bird's nest in the bobbin area of a large embroidery machine