On their site about the Modalsatz, canoo.net consideres both, "außer" and "insofern" as conjunctions that introduce a Restriktivsatz, while I think they are completely different.
I can understand "insofern" restricts the meaning of the first sentence, but "außer" is not restricting . As I understand, it rather means: if the second sentence is correct, then first sentence will not be correct, for example:
Sie können das Spiel nicht mehr gewinnen, außer wenn sie ein Wunder schaffen.