problem with my Plesk Panel+admin mysql

[massilia]

I have a problem with my Plesk Panel.
When I'm logged with "admin" I can create a database at will but I got the following error message when I want to add an user :
Error : database connection failed because the account doesn't have administrator rights :
Access denied for user 'admin'@'localhost' to database 'blogtest'

I looked for admin user rights in phpmyadmin and he has all the rights except Grant right...
I tried to add this right from phpmyadmin but I got the following error :#1045 - Access denied for user 'admin'@'localhost' (using password: YES)