Difficulty adjusting tension to avoid bobbin looping

This likely is related to thread tension. You might increase that adjustment a bit higher than the book settings. If that does not fix the problem, here are some additional considerations?

https://www.thesprucecrafts.com/fixing-bobbin-thread-bunching-looping-tangling-2978069

Thread Breaking Bunching Looping and Tangling All Problems ONE SOLUTION...

Usually, one will adjust the bobbin tension to accommodate heavier thread. If one does this often, a separate bobbin case set up for heavier thread is handy.

There are some links posted here:
Lower thread tension too loose Sewing Machines
that may be helpful in making those adjustments.

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The bobbin has a tiny screw on the side that is the tension setting. Turn it a slight 1/8 turn and test (a small turn makes a big adjustment). Be sure to mark the original setting of the screw before making any adjustments so you can back out the adjustments if it's not working.

Bobbin Tension How and Why Tension is Adjusted

Understanding Bobbin Tension

Understanding Thread Tension Threads

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Could be several things.

First, install a brand new needle, perhaps a top-stitch needle will work better because it has a groove in the shaft that protects the thread. Make sure the needle is the appropriate size for the thread you are using--a needle eye that is too small will cause the thread to hang or shred. When the needle penetrates the sandwich, the top thread forms a loop. If the eye is too small, the thread will not come back up through the sandwich, leaving a loop under the fabric.

Remove the top thread from the machine.
ALWAYS RAISE the presser foot and rethread from the beginning.
Set the top tension to the midway point and retest your machine on a quilt sandwich made of the same fabric layers as your project.. Tension problems are more evident when sewing multiple layers of fabric, so tweak the tension until the top and bobbin threads meet in the middle of the sandwich.

I don't believe you will have to adjust the bobbin tension, but in case you do, be very careful. Mark the tension screw setting FIRST so you can reverse any changes you make. Bobbin adjustments are made in very SMALL increments. Because I tend to use a lighter weight thread for my quilting, I have a specific bobbin case that is adjusted for my quilt thread and I keep my other bobbin for regular sewing. Then I don't have to keep adjusting the bobbin tension when going from sewing a quilt to something else.

Some machines do better with certain brands of threads than others.

Adjusting Sewing Machine Tension to Prevent Problems

Has the take up lever been replaced with the new updated one that does not let the thread slip out?

I have a Sapphire 835 and am having the same problem with all the looping under the fabric. It DOES NOT do this while doing any type of sewing other than Free Motion Quilting. When I FMQ, I end up with a disaster. It is very frustrating, as that was the whole purpose for this machine. I have an older Viking and love it. So disappointed and don't feel that I should have to adjust/fix/fiddle with anything.

Try the 5 page tutorial....HERE

I had this same problem, on my Designer Diamond. I blew it out with an air can and that didn't work, but then I decided to turn it off, then back on and start sewing a line in reverse. That seemed to fix it. My machine is working fine now.

it is a problem your tech can fix, ( should have been done when he had the covers off) but take it back in and tell him to adjust the sensor on the cutter so it doesn't rub and to clean the sensor. this will fix the problem

Thread tension. Beware of adjusting the bobbin screw. Pfaffs maintain their tensions just fine without adjusting the bobbin tension, unless you're using some crazy thick or fine thread. If you must change your bobbin tension, you would be well-advised to buy an extra bobbin case, because re-calibrating your tensions after changing them can be less than fun.

Increase your top tension, and your looping should cease. I normally keep my tension around 4.5. Also make sure you're using a good quality thread, and that there's nothing in the upper tension assemblies. Sometimes a tiny piece of debris can keep the top tension from applying enough tension, so turning the machine off and blowing some canned air in there can help as well.

Good luck,

-R