Background: There are two sites running on the same domain. One in the domain root (http://domain.com) and another one running in a folder (http://domain.com/english). The site in the domain root is built in Swedish, the site in the folder is in English. Each site has its own, independent WordPress installation and its own table prefix in a shared MySQL database. Comments are not used in blog posts, only in a "guestbook" page.
Problem: Comments are working perfectly on the site in the domain root, the Swedish main site. However, using the same installation files and the very same theme, comments are NOT working in the English site in the subfolder.
Question: It is not a theme issue, as comments are not working in the default theme either. It is not an incorrect setting. It is not a mis-typed comments.php or include as it works well on the other site. What could possibly cause the comment feature to die completely like this, when it is basically two identical sites? Can it be the fact that it is installed in a folder? If so, what can I do about it to get the comments to work?