Please note, that simultaneous operation of UCS and Debian on a UEFI system is not supported starting with UCS 5.0-0.
The reason for this is the GRUB boot loader of Univention Corporate Server, which partly uses the same configuration files as Debian. An already installed Debian leads to the fact that UCS cannot be booted (any more) after the installation of or an update to UCS 5.0. A subsequent installation of Debian will also result in UCS 5.0 not being able to boot.