Chinese to English course's Question Threads sometimes act funky
When you get a question wrong, you get the option to discuss it.
Sometimes when you click discuss, something like this pops up.
You then submit your comment and are either redirected to the actual comment page, or taken to a page that gives you a 404 error. This link is the latter case.
Huh, I've also seen a page like that when seeing a discussion topic for the Russian to English course. At first I thought Duolingo redesigned the forums. Whatever it is, it doesn't just happen for the Chinese to English course.
This is how all empty sentence discussions appear to me in all courses since the new website was implemented. As I've not started any new discussions since then, however, I don't know if they are working as they should.