@Markus Wilhelm
I have done 100's of Kerberos setup on hadoop never encountered this particular error weird
To be after to help you can you share a scrambled version of the below files
/var/kerberos/krb5kdc/kadm5.acl
/etc/krb5.conf
/var/kerberos/krb5kdc/kdc.conf
Can you ensure that all the components of your cluster BIKW are running? Validate the services are okay with "Run service Check"
Make sure the below daemons are set to autostart and are running before launching the Ambari Kerberos Utility
# chkconfig krb5kdc on
# chkconfig kadmin on
Please revert