I've had some thread jamming problems with my 401. It, too, is in the middle of the seam. You have to cut out the thread jam to get the fabric out of the machine. Usually, by then, you have no clue what actually happened. It appears as if the top thread loop just gets longer and longer with each stitch until it gets hung on it's way around the bobbin. I cleaned the machine extremely well in the bobbin area following the directions on this video:
https://www.youtube.com/watch?v=4cB50JdKh8g. After cleaning, the problem almost disappeared, but not entirely.
bkay