The installation of an app takes several hours and does never end
No app is installable. Whatever app you are trying to install, you are getting the same endless installation.
When doing the installation on the command line with
univention-app you will see the following output. You might see the same output when checking
33060 packages 19-03-05 22:47:47 [ INFO]: Media change: please insert the disc labeled 33060 packages 19-03-05 22:47:47 [ INFO]: 'UCS Linux 4.3 GNU/Linux 4.3-3 _Ucs433_ - Official amd64 DVD Binary-1 20181209-22:04' 33060 packages 19-03-05 22:47:47 [ INFO]: in the drive '/media/cdrom/' and press [Enter]
Remove all previously added lines in your file
echo > /etc/apt/sources.list apt update
Restart the install process then.