I'm assuming you have a 7700. I do and I get this error all the time when switching back and forth between the APC (plate converter) and regular stitching. Here's what I do - I turn the machine off. I lower the needle to the lowest position. I turn the machine back on. I turn the hand wheel to raise the needle and bring up the bobbin thread. The LCD display now shows the steps to change from the plate converter to regular sewing or vice versa. I follow the steps, holding the bobbin thread tightly. It generally clears the error message at this point, but if it doesn't, I follow this procedure again.
I believe it's caused by a sensor that's telling the machine there is thread still in the path from the bobbin to the plate converter, but I'll be darned if I (or my tech) can find it. My dealer is pretty clueless and has been no help.
If this doesn't work, you might want to try removing the bobbin before you switch from APC to regular sewing or vice versa.
If I didn't love the rest of the machine so much, I'd have traded it in!
Please let me know if this helps.