I have one forum that only available for registered user. So when guests click the forum link, they will redirect to login pages.
Then, I found the problem...
After login from that pages, user can't redirect properly. There is an error message "The forum you selected does not exist.". The pages redirect to: -http://xxx/phpbb3/viewforum.php?start=&sid=xxx
This case only happen when I activated Forum ID removing.