Problem
You have configured a policy to set ucr variables but they do not apply on UCC.
root@vm-ucs:~# univention-policy-result -h vm-ucs.school.dom -D $( ucr get ldap/hostdn ) -y /etc/machine.secret $(ucr get ldap/hostdn)
[...]
Policy: cn=school-ucc-ucrsettings,cn=ucc,cn=policies,dc=school,dc=dom
Attribute:
univentionCorporateClientDesktopEnv;entry-hex-6b65726265726f732f6b6463
Value: vm-ucs.school.dom
Solution
To set ucr variables do not use policies of type “UCC Desktop-Settings” but instead use a policy of type “Univention Configuration Registry”.
The first do apply to environment variables and therefore do not get applied to ucr variables.