This is a bit dated, the OP (original poster) took their machine in for a tune up and it was working fine after that.
I have to take my modern 820 to the shop (using the same model number for two completely different machines is just not sensible!), I used it hard the past year and it is also due a tune up. Another member here on the board had the exact same thing going on with their machine. Basically both of our machines worked fine one day and then without changing anything they didn't the next. I've done most of the maintenance type things I can reach. I'll look for that thread to see if they fixed it themselves. People reported in that although the error message was showing in one place, it could be a thread in the bobbin area, but again, I did all the normal cleaning and resetting things. My car was in the shop for almost three weeks and then it was Christmas and taking it in is on the list of things to do soon.