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The version of XOOPS doesn't matter, because its not XOOPS thats returning the 403 forbidden message, its Apache.
Normally, the "You don't have permission to access /tuis/modules/newbb/post.php on this server" message means that the file is chmod'ed (file permissions) to 0640 or something like that. Just to be thorough, verify that the file is chmod'ed 0644, otherwise known as rw-r--r--