If you get an error from the pre-update-checks-x.x-x command like this one the following steps might help:
minimum_ucs_version_of_all_systems_in_domain:
The following extensions are incompatible with UCS 5.0:
cn=63openvpn-sitetosite,cn=ldapacl,cn=univention,dc=ibb-tga,dc=local: [unspecified..unspecified)
Step 1: Verify if some packages are outdated and have been replaced. In this example the openvpn4ucs package replaces old packages. First you should remove the old packages (if any):
apt remove python-univention-openvpn-common univention-openvpn-master univention-openvpn-schema univention-openvpn-server openvpn
Step 2: Execute the following command to delete the extensions of the old package:
source /usr/share/univention-lib/ldap.sh && ucs_unregisterLDAPExtension --acl 63openvpn-sitetosite
After these steps, the update check should proceed without issues.