Principiī autem nūlla est orīgō; nam ex prīncipiō oriunt omnia, ipsum autem nūlla ex rē aliā nāscī potest (nec enim essetid principium quod gigneretur aliunde).
I understand that “res” is omitted, and it would be: nūlla res ex rē aliā nāscī potest. Therefore, nūlla is nominativus, but in which case would be “ipsum autem”? Could it be an errata and nūlla is nūllā in ablativus and ipsum autem is nominativus?
Ipsum is nominative, and its antecedent is principium. As for nulla ex re alia, I checked Ørberg, and I think there’s a mistake in the text. It should be ‘nullā ex rē aliā’, all taken together. So ‘but/while it (itself) can be born of nothing else (lit. out of no other thing)’.