Apparently they do a whole lot of blocking and deleting of websites in response to complaints in Germany. I don't know if teacup.social was among them but with that much stuff getting nuked every day there are bound to be a few false positives. Something to consider before you host anything at Hetzner, perhaps.

Edit: On further searching the web, it seems that ordinary web censorship doesn't typically involve big scary-looking notices from the BKA. Maybe they happened to somehow or other share an IP address (or a server) with these guys?