Hello can you try in other installation, as there are lots of plugin installed, so someone is adding extra rule before every anspress rule, which is a cause of this error. I will suggest you to try in other fresh install (or existing install) but keep in mind that installing other Q&A plugin in same site may result the same. cause we use same post type and rewrites rules for posts.
here is the fix for rewrite rules. https://github.com/open-wp/anspress/archive/master.zip install it manually and let me know asap