I also have a Brother ( QC1000) and it suddenly started shredding thread right after a checkup. I tried everything that was mentioned in these posts, called the repairman and was completely going nuts. The machine was able to handle anything I threw at it thread-wise before. I won't detail what I went through for about 3 days before I found the problem, but it was a burr on the inside of the plastic BOBBIN....not the case, the bobbin itself. The plastic bobbins are evidently put together in two pieces and on the flat part of the plastic on the inside of the bobbin there was a burr at the seam. Impossible to see or feel unless you are looking for it. I figure the repairman used a generic plastic bobbin with their own thread to test the machine and that's where it came from. Changed the bobbin and I've been cruising along ever since.
Good luck, I know how frustrating it can be!