ncdu (Terminal-Tool für die Speicherplatzverwaltung):

mkdir /boot/tmp
cd /boot/tmp
wget https://updates.software-univention.de/4.3/unmaintained/4.3-0/amd64/ncdu_1.12-1+b1_amd64.deb
dpkg-deb -x ncdu_1.12-1+b1_amd64.deb .
The system can not be updated to UCS 5.0 due to the following reasons: 
python_ucr_template_compatibility: The following UCR templates are not 
compatible with Python 3: 
/etc/univention/templates/files/usr/share/univention-web/js/umc/hooks/appliance.json

Lösung:

sudo wget https://raw.githubusercontent.com/univention/univention-corporate-server/5.0-0/base/univention-app-appliance/conffiles/usr/share/univention-web/js/umc/hooks/appliance.json -O /etc/univention/templates/files/usr/share/univention-web/js/umc/hooks/appliance.json