Roundcube 0.6 plug-in password

[Agrum]

I encountred a problem with "password plug-in for roundcube" When I am logged, I cannot change my password, I have this error's message:

Could not save new password

main.inc.php and db.inc.php appears correctly configured

$rcmail_config['db_dsnw'] = 'mysql://GoodUser:GoodPass@localhost/roundcubemail';

in var/www/webmail/logs/ error 's file I have this message:

[03-Apr-2012 09:18:44] MDB2 Error: not found (-4): unable to find package 'MDB2_Driver_mysql

MDB2_Driver_mysql, appear correctly installed... Pear list:

MDB2 2.5.0b3 beta MDB2_Driver_mysql 1.5.0b3 beta MDB2_Driver_sqlite 1.5.0b2 beta

Can you help me to solve my problem ? Many thank's in advance