[sork] Cannot connect to external MySQL server

[Seba]

Version: Passwd 3.1.1

I use Horde/IMP on a webserver with an own mysql database. User
accounts and passwords are stored on antoher mysql server. I've
configured Passwd so far but I when I try to change the password of an
user I get the message;

Unable to connect to SQL server (Verbindung zum SQL Server kann nicht
hergestellt werden).

All needed pear modules should be installed like PEAR,Mail,Log,DB...

>From this webserver I can connect to the remote MySQL server with the
mysql client (command line) without problems but not with the passwd
application. The backend.php seems to be correctly configured.

   'params' => array(
        'phptype'    => 'mysql',
        'hostspec'   => 'ip of the external server',
        'username'   => 'hordepwd',
        'password'   => 'password of the user',
        'encryption' => 'plain',
        'database'   => 'mail',
        'table'      => 'mailbox',
        'user_col'   => 'username',
        'pass_col'   => 'password',
        'show_encryption' => false

On the remote sql server there's no firewall active and the mysql
server is listening on port 3306.

Thanks,
Seba