Some error after installing

Hi everyone ,
I own a web hosting site , and my clients have problems installing wordpress ,
They get this error message after installing , when they try to reach administration control panel of wordpress :

Fatal error: Cannot unset string offsets in /var/www/get-start.knd.ro/wp-includes/widgets.php on line 358

First i believed that they are doing something wrong so i tryed myself and got same error message …

The php version on the server ( running slackware linux ) is 5.2.5 , the apache version is 2.2.8 and mysql server 5.0.51a …

So can you tell me why is this happening ?
Regards!