I have checked file permission, configuration etc, but I can't figure out why this happens.
config.php file to add the hidden variable:
ExpressionEngine has a number of, not needed frequently, but available for use variables, see Hidden Variables.
One of these should work, assuming there wasn't some odd bug in EE 2.5.3. Is there a reason you haven't updated to 2.5.5? You may also want to upgrade.