In preview 9, we switched from using <CascadingAuthenticationState> in the templates to use <AuthorizeRouteView> inside the <Found> part of the <Router> config.

Unfortunately this doesn't account for the <NotFound> part of the <Router> config, which now doesn't have any cascaded authentication state. This leads to an exception if you visit a non-existent page, because the main layout tries to use an <AuthorizeView>.

Fix

In the template's App.razor, go back wrapping a <CascadingAuthenticationState> around the whole thing.