The site was built using Joomla! 1.5, so I enabled debugging mode and set the error reporting level to maximum to know what is the problem.
Then this error was displayed when opening any page:
Parse error: syntax error, unexpected $end in filename:lineno
it was the index.php file of the template, and after some debugging I found that the problem is the using of short php tags, like this:
<?
The server was not configured to allow short tags.
For maximum compatibility, it is preferred to use full tags, like this:
<?php.
Hope this help you all!