I have a quilting model Pfaff also, but a different model number. Maybe they have some features in common.
When setting up for FMQ, the take-up lever needs to be set at the 'between level'.... by that I mean all the way up to lift the pressure foot, all the way down to lower the presser foot, and a mid-way setting for FMQ. This sets the quilting foot to just sit on top of the fabric, enough to glide over, but not put on any pressure. To set it to the midway level, I need to push the lever to the rear as I lower it.
I found that if I didn't have the take-up lever set to this midway point, the tension would go out and the thread would break. Maybe your quilting model has the same system?