It looks like your htaccess is customized from the default Pligg version the way your URLs are formed. Maybe something caused by the customization caused this?
Just to recap my settings that seem to be working on the
Pligg Demo:
- Copy the categories code provided from the bottom of the category admin page to your htaccess.default files
- Rename htaccess.default to .htaccess
- Turn on URL Method 2 from Admin > Configure > UrlMethod
- Set Friendly URLs as true (default setting) in Admin > Configure > FriendlyURLs