Error when I attempt de configure a new domain parameters

[devinas]

After creating a new domain on plesk 8.1, I pass to the next step. When I finish to put all parameters required and after validation, I get the following error message:

Imposssible de mettre à jour les préférences d’hébergement (Imposssible to update hosting’s preferences). Update FTP user failed : ftpmng -–update-user failed. Virtual host FTP on IP 91.121.88.42 already exits.

Somone could help me solve this problem ?

Best regards