Hi Flavio,
If the issue is fixed when you log off the private server, the possible cause is that the 'Force custom server security' checkbox is enabled for that user in RU Server settings (Admin Console -> Address Book Manager -> User details -> General):

If this checkbox is enabled AND if Viewer is signed in on a private server, Viewer always tries the 'Custom server security' authentication method, ignoring other auth methods.
Hope that helps.