Comments wont post in twenty fifteen

My blog is https://www.trugundschein.org. I recently updated to the twenty fifteen theme because of a problem with a genesis theme. Multiple users on different machines and browsers have tried to add comments but they do not submit. Earlier comments are still present. I can register new users: that did not change anything. I tried changing the settings for how Spamhammer and the site more generally deal with comments. no change. I deactivated all plugins to try to see if that was the problem but it did not change anything either. Any suggestions from anyone with more experience than I?