I can tell you that that isn't in the service manual. LOL!
Sometimes the stitch length is great for moving forward, but too close to move backwards. So you could try opening the stitch length just a "skosh."
So the first question I have to ask is, are the feed dogs high enough to grab the fabric and pull it backwards? When the needle is in it's highest position, the bottom of the feed dog teeth should be level with the needle plate. In other words, the entire tooth should rise above the needle plate.
Also, how tight is your presser foot tension? When you lift the presser foot lever, then drop it, does it slam, or does it limp down?
Maybe the first thing I should ask is, are you using the #3 buttonhole presser foot? That makes a huge difference!
Are the feed dogs moving when you are in the backwards zig-zag? Take the presser foot off and watch what they are doing. I honestly doubt that the problem has anything to do with the buttonhole mechanism.
Let me know what you think...