HILFE bei Fehlermeldung: failed to connect to MySQL (Datenbank ist von 1&1)j

Hallo, erhalte bei der Installation folgende Fehlermeldung:

Failed to connect to your MySQL database server. MySQL reports the following message: Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2).
Are you sure you have the correct username and password?
Are you sure that you have typed the correct database hostname?
Are you sure that the database server is running?
For more help, see the Installation and upgrading handbook. If you are unsure what these terms mean you should probably contact your hosting provider.

Habe in der default.settings.php (/sites/default/) folgende Änderung vorgenommen:

* Database URL format:
$db_url = 'mysql://dboxxxxx:passwortxxx@db1321.1und1.de/db249563589';
* $db_url = 'mysqli://username:password@localhost/databasename';
* $db_url = 'pgsql://username:password@localhost/databasename';
*/
$db_url = 'mysql://dbo24xxxx:db1xxx.1und1.de/dbxxxx';
$db_prefix = '';

erhalte nach wie vor die fehlermeldung.
was kann ich nun tun? bin total verzweifelt.