The first thing I would check again is the bobbin. Are you sure it is inserted into the bobbin case with the thread coming off in the correct direction? Compare it against the manual. It's really easy to miss something as basic as that.
A charitable organization once brought me a machine that had started doing what you describe -- for no apparent reason. They couldn't afford to take it to a technician. The machine was a classroom machine for a sewing class. Because the machine worked fine before a bobbin was changed and started breaking thread after the bobbin was changed, I started looking at the bobbin. Sorted through my box of spare parts and tried another bobbin. The machine worked fine! Compared the two bobbins and they were ***almost*** the same; only a very slight difference in shape. What had happened in the class is that the student had grabbed another bobbin to wind, and it was not the correct bobbin for the machine.
You mentioned that you wound another bobbin and tried that. Was it a bobbin that had stayed at your home? I would still try a third bobbin in the machine. Although plastic bobbins are more likely to deform, even metal bobbins can deform just enough (for example, from a drop on the floor) to cause the thread to break. And, occasionally, it's possible for a package of bobbins to all (or mostly) come from the manufacturer defective.
Edit: If it's not the bobbin, then the next thing I would examine carefully is the bobbin case. Check especially for a tiny piece of lint caught underneath the spring on the case. You can use a large needle to carefully lift up the spring to clean under it. Jamie Wallen shows how to do this within the first minute of his video on longarm tension (incidentally, his method works well for domestic machines also in terms of adjusting tension for quilting rather than piecing):
https://www.youtube.com/watch?v=Q1mRhcquZTM
You mentioned that the thread breaks at the bobbin case. That would be right where that spring is. A piece of lint can cause the tension on the bobbin thread to be tighter than is set by the screw.