I have finally fixed my wife's Viking 100 that had exactly this same fault. The main issue was a cam that had rotated out of alignment (probably due to a jam or a broken needle blocking a mechanical part). The cam is in the base of the machine and is held in position on the main rotating shaft with an allen key fixing. The cam sits inside an upside down u shaped piece of metal. The piece of metal drives the feed dogs (which pushes the material along as you sew). I found this cam had rotated out of position by almost 180degrees. By loosening the allen key, rotating the cam and retightening it the machine now goes the right way! I've tried to paste in a photo but don't know how to or if its even possible on this forum.

so annoying, my janome was doing this, I just frantically messed with all the setting and luckily something clicked and it worked again, mine isn't digital though. hope you get/got it sorted

Did you get this solved? What was the solution?