first thought that came to my head... could the 'sensor' be picking up some lint or snip of thread, that is 'throwing off the machine', some of our machine can be oh so sensitive. If you have 'canned air', try blowing out the threading area carefully. Also, somewhere in my memory banks, I remember about threading or clearing the disc area with dental floss to clean sewing machine.
Before you re-thread AGAIN try a good clean.
If all else fails, then it might be time to see the DR. (machine, not you)
