Hi foto50,
It seems to happen in situations where the Joomla! username is unique, but the OpenSim name is taken. When a Joomla! username is taken it returns the error: 'The username you entered is not available. Please pick another username.', but when the OpenSim name is taken it returns: 'Registration failed: Registration failed: ". No account for either Joomla! or OpenSim appears to be created either way, but I will test it further.