Configuration in Multisite, login to main page not possible anymore


on a multisite installation I can't login anymore. It does not seem to be an issue of the password, this is correct and already changed several times. Rather it seems to be a misconfiguration. To one of the subpage I can login successfully, but when I try to get to the main-page, the login-screens appears... It seems to me, as if the site tries to redirect to the login-screen always, but since I don't get into the backend anymore, I can't check or change something.

Is there a way to get to the backend again and check configuration?