It's usually the tiny hook in the needle threader that gets out of whack. You can push it back or open it carefully with a pin.
If you don't have a straight stitch plate, try covering up most of the zigzag hole with paper and tape, post its, - we used masking tape. It helps. The single stitch machines that people use for FMQ have the single hole.
I've got a 2170 and have played with the newer Pfaffs and don't want one. I prefer the vertical bobbin rather than the drop-in. They are nice otherwise.