Problem
Uninstalling some UCS packages will result in an empty /etc/apt/sources.list which means a reinstall is not possible due to missing sources.
This might happen if uninstalling openssl or univention-update
Solution
Create a valid /etc/apt/sources.list by hand containing the needed entries matching your UCS version and architecture:
deb http://updates.software-univention.de/4.0/maintained/ 4.0-0/all/
deb http://updates.software-univention.de/4.0/maintained/ 4.0-0/amd64/
deb http://updates.software-univention.de/4.0/maintained/ 4.0-1/all/
deb http://updates.software-univention.de/4.0/maintained/ 4.0-1/amd64/
deb http://updates.software-univention.de/4.0/maintained/ 4.0-2/all/
deb http://updates.software-univention.de/4.0/maintained/ 4.0-2/amd64/
deb http://updates.software-univention.de/4.0/maintained/ 4.0-3/all/
deb http://updates.software-univention.de/4.0/maintained/ 4.0-3/amd64/
deb http://updates.software-univention.de/4.0/maintained/ 4.0-4/all/
deb http://updates.software-univention.de/4.0/maintained/ 4.0-4/amd64/
deb http://updates.software-univention.de/4.0/maintained/ 4.0-5/all/
deb http://updates.software-univention.de/4.0/maintained/ 4.0-5/amd64/
deb http://updates.software-univention.de/4.1/maintained/ 4.1-0/all/
deb http://updates.software-univention.de/4.1/maintained/ 4.1-0/amd64/
deb http://updates.software-univention.de/4.1/maintained/ 4.1-1/all/
deb http://updates.software-univention.de/4.1/maintained/ 4.1-1/amd64/
deb http://updates.software-univention.de/4.1/maintained/ 4.1-2/all/
deb http://updates.software-univention.de/4.1/maintained/ 4.1-2/amd64/
deb http://updates.software-univention.de/4.1/maintained/ 4.1-3/all/
deb http://updates.software-univention.de/4.1/maintained/ 4.1-3/amd64/
deb http://updates.software-univention.de/4.1/maintained/ 4.1-4/all/
deb http://updates.software-univention.de/4.1/maintained/ 4.1-4/amd64/
deb http://updates.software-univention.de/4.1/maintained/ 4.1-5/all/
deb http://updates.software-univention.de/4.1/maintained/ 4.1-5/amd64/
deb http://updates.software-univention.de/4.2/maintained/ 4.2-0/all/
deb http://updates.software-univention.de/4.2/maintained/ 4.2-0/amd64/
deb http://updates.software-univention.de/4.2/maintained/ 4.2-1/all/
deb http://updates.software-univention.de/4.2/maintained/ 4.2-1/amd64/
deb http://updates.software-univention.de/4.2/maintained/ 4.2-2/all/
deb http://updates.software-univention.de/4.2/maintained/ 4.2-2/amd64/
deb http://updates.software-univention.de/4.2/maintained/ 4.2-3/all/
deb http://updates.software-univention.de/4.2/maintained/ 4.2-3/amd64/
deb http://updates.software-univention.de/4.2/maintained/ 4.2-4/all/
deb http://updates.software-univention.de/4.2/maintained/ 4.2-4/amd64/
deb http://updates.software-univention.de/4.3/maintained/ 4.3-0/all/
deb http://updates.software-univention.de/4.3/maintained/ 4.3-0/amd64/
deb http://updates.software-univention.de/4.3/maintained/ 4.3-1/all/
deb http://updates.software-univention.de/4.3/maintained/ 4.3-1/amd64/
deb http://updates.software-univention.de/4.3/maintained/ 4.3-2/all/
deb http://updates.software-univention.de/4.3/maintained/ 4.3-2/amd64/
deb http://updates.software-univention.de/4.3/maintained/component/ 4.3-2-errata/all/
deb http://updates.software-univention.de/4.3/maintained/component/ 4.3-2-errata/amd64/