Many Janomes had this problem. Janome has a latch on the take up levers now. They know about this problem. So far they have not designed a fix for existing machines
I thread my take up lever from the left side...no problem.
BTW you have to physically lift up your pressure foot before threading, not just look. This will open the tension disc to allow your thread to rest fully inside the tension discs.
I Love my 6600's. One at home, one at the cabin.
best wishes for a good fix for your issue.