[eluser]WanWizard[/eluser]
APPPATH points to the application directory, correct. If you use it you don't have possible relative path issues.
If you are sure the path is correct, it must be a rights issue. Check every directory, starting from the root, to make sure your apache user has X rights on every directory, and XW rights on '/opt/lampp/htdocs/www/CI_forum/application/controllers/'.