Exchange 2013 error 500 by accessing ECP as admin

I have a small Exchange 2013 (SP1) site with just one CAS.

By accessing of the CAS, OWA runs properly for each user and also the options-page is well available – not so by accessing it as "admin" because of the "error 500". The same occurs by accessing ECP directly viahttps://cas/ecp: after passing of credentials the normal options-page occurs for each user – not so for the admin.