It's possible to use Sharepoint2013 by permitting a user directly on the site. I can either put it into a SharePoint group or give it the appropriate permissions directly. But if I add a AD security group to the site security settings and give that group FullPermission, no member of that AD group is able to log in properly. The SSO is set into place, the user gets authenticated - but not authorized. I know about the difference security group <> distribution group (which will not work in SharePoint). Any Ideas?
As addendum: SharePoint server and client are located in the same ActiveDirectory domain.