I really had a strange error when I was recently browsing to my web site. The nginx web server returned a 400, which only happened to me with my main browser: Mozilla Firefox.
After investigating, I figured, that it was a cookie which was sent by my browser and apparently set by Kupu during editing. If you find this weird behaviour for your site as well, check the cookies. The cookie name which led to my problem was: initial_state. Delete it and you should be able to browse to wherever you run into this error first.