When you try to login with SCLogin on this page, does it work?
gesundheitsanbieter.de/anmelden
If that works, then SCLogin is working.
On the jbusiness directory pages, can you place a standard Joomla mod_login module and test if that has the same issue, on those pages? If so, then the issue is something else and not related to SCLogin.
If it the Joomla login module works, please leave it enabled and let us know what page it's on. Also, please post test (not admin) credentials that we can test authentication with.
Thanks,
Alex