After intensive investigation on the "No client with ID $client_id found" problem. I came to an conclusion.
The error happens because ISPConfig is looking for the associated client associated with the sysuser of the email and when just the sysuser exists, than the error appears.
When a "CP User" (sysuser) gets created, it doesn't has a associated client, and when the sysuser created the email mailbox the error appears.
When a client gets created, the sysuser get automatically added to the system.
So this sysuser has now a associated client and the error won't appear.
1.)So the best way, would be to just create the missing client for the sysuser.
2.)Whereever the error appears, go in the appropriate ispconfig3_xxxx.php file, located in the plugin folder of the roundcube plugin folder, search for update and then search for sysuser_id.
Should be something like that
$update = $client->mail_spamfilter_user_update($session_id, $id, $mail_user['sys_userid'], $params);
and replace it with a 0,
$update = $client->mail_spamfilter_user_update($session_id, $id, 0, $params);
PS: French translation is in the svn.