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if a new user try to register and the first site is filled out --> Click at "Next" --> then the Login page appears
Can you please try to register at my page and look for the registration error?
that because your set main menu homepage / jomsocial home page as registered access level. please goto menu manager, and set jomsocial home page as public at main menu.
you can hide activities stream from public. from jomsocial backend > configuration > layout > Show Activity Stream = members only.