Thanks for the info. I found the locked lever, but to my great astonishment, all my thieves tools have gone, so there's no one who can pick this lock.
Not long ago, I had at least 2 lockpicking tools set per person, and I cannot remember having missed 8 lockpicks. Another great mistery.
I went back to an old save and will try if I can do the puzzle again. This old save also has no lockpicking tools anymore, so hope it works.
