I noticed that the server had the wrong name. Renaming was done very quickly but the name was also used by LVM. To rename the LVM you need to umount all volumes. This is not possible because all binaries are on the LVM. I decided to re-install the server. This is faster than trying to find the solution. So I did. For tomorrow evening I plan to configure DNS and DHCP. Since I already know how I will also install OpenLDAP. I than only need Squid and then I am ready to activate my DMZ.