With UCS 5.0-3 errata update 794 Security and bugfix errata for Univention Corporate Server a dependency problem has been introduced, that can cause the Samba4 / Active Directory-compatible Domain Controller packages to be removed during the update. This could happen if errata updates were installed on UCS 5.0-3, or if the system was updated to UCS 5.0-4
A fix was provided with UCS 5.0-3 errata 795 Security and bugfix errata for Univention Corporate Server.
To find out if the server affected, check if the output of univention-app update does not contain the app samba4, even though it should be installed.
If your system is affected, please reinstall the app in the following way:
apt-get update
univention-app install samba4