filing so that this issue is not lost
attempt to log in as a valid user with invalid password, then log in with the same user.

frontend shows error messages :

"Undefined index: debug_mode[/usr/local/apache2/htdocs/zabbix-1.7.2/include/config.inc.php:455]
Object of class CJSscript could not be converted to string[/usr/local/apache2/htdocs/zabbix-1.7.2/include/config.inc.php:456]
Object of class CJSscript to string conversion[/usr/local/apache2/htdocs/zabbix-1.7.2/include/config.inc.php:456]
Object"

(trimmed like that).