It sounds to me (by the location of the spring) that it does have something to do with the bobbin winder. I have the Juki 2010Q, which is very similar in design to your machine. I had a minor panic one day when mine wouldn't sew, and it turned out the bobbin winder had been engaged by mistake. Once I popped it back into place, all was well. So that's my guess.... for what it's worth.
