Problem with User registration

if customer wants to register, this warning will appear:

The username you entered is not available. Please pick another username

With SMTP Mailing, the registration will confirmed, the new user ist in the database.
But this warning ist not so nice.

Is there any solution? Joomla 3.1.5

thanks
uws

You can put different translation for that text in language ini file if only text is problem.I think it is in:

languages/en-GB/en-GB.com_user.ini