I have Xenforo installed in the root directory and I just installed WordPress in /articles sub directory. Well, everything is working fine using the WordPress default URL structure but when I am changing it to domain.com/articles/%postname%/ the WordPress post is getting redirected to Xenforo's 404 page not found.
If you temporarily remove XF's .htaccess file, does it work? I would presume no, which likely points to an issue with the WP .htaccess configuration. XF won't be involved in any way if there's an articles sub-directory (unless there's a 404 within that the articles/.htaccess file doesn't grab).