Thanks for posting this. It makes me feel not so alone in my sewing room when things go whacko. I had a very similar experience a few days ago with my 40 year old Bernina 830. I broke a needle somehow and after I replaced it there were lots of snarls and thread breaks. Eventually I noticed the bobbin tension screw was loose and fixed that, but still it was not making proper stitches. I could type a long list of things I tried, including trying to use a Diamond Deb nail file to remove the nick in the needle plate that happened when the needle broke. I finally noticed the thread had jumped out of the take up lever. Hitting myself in the head because on that machine it's right out in full view and probably the easiest and most logical thing to check. My excuse is it never happened to do that before. Lame, I know.
The odd thing is that I also use my mom's Elna 62C that's a little older than the Bernina. On it, the thread goes through a hole on the take up lever and can't jump out, but the last guide above the needle on the Elna is designed such that the thread hops out of it with the slightest provocation. The corresponding guide on the Bernina is very effective. Wouldn't it be handy if everything was interchangeable and we could put them together so they each had the best features?
Oh, and of course I was sewing a 10-minute block at the time of my Bernina fiasco. Guess how long that one took!