We had a short interrupt in our DNS server which caused some systems to prevent logging in.
We have resolved this issue and may plan a further update window of our DNS infrastructure to avoid this issue.
We have checked every DNS related parameter and changed some of them to static values.
We think that this resolves the issues and will continue to monitor during the next days.
We have noticed that other servers, which haven't been affected in the last couple of hours, show similar DNS / DHCP behaviour.
We are investigating the issue.