I had done this on my 6500, and it's very similar to the 7700, which I now have. One thing to check is the bobbin case itself. When this happened to me, while it was jamming the needle made several stabs on the bobbin case. They were on the flat area towards the back of the case where you would think it wouldn't make it difference, but it does. My husband sanded off the plastic that was sticking up as a result of the jabs and all was well. I believe the bobbin thread was getting snagged on those little pieces sticking up. As soon as I could I replaced the bobbin case.
I would also check to make sure you didn't break off a small piece of your needle and check your threading again just to make sure the thread is properly seated. I know you said you did that, but one more time can't hurt.
I'd also check to see if there's not some small piece of thread hanging around in the bobbin area or feed dogs.
Hope this helps!