Thanks for the feedback. Such thing is possible, since my dev. environment runs php 5 and mysql 5, but I really don't understand where could be the problem. We just delete and insert a row in the database in captcha.php
mysql_connect($host, $user, $password) or die(mysql_error());
mysql_select_db($db);
query($dbprefix, "INSERT INTO jos_comment_captcha (insertdate, referenceid, hiddentext) VALUES (now(), '" . $referenceid . "', '" . $textstr . "')");
query($dbprefix, "DELETE FROM jos_comment_captcha WHERE insertdate < date_sub(now(), interval 1 day)");
Thank you again. I will write a warning message to the new users.