Hi,
try to run the join script again which usually creates the machine.secret:
univention-run-join-scripts --ask-pass -dcaccount administrator --force --run-scripts 10univention-ldap-server.inst
If this does not work, do the following:
password="1234QWERasdf"
udm computers/domaincontroller_master modify --dn "$(ucr get ldap/hostdn)" --set password=$password
echo -n $password > /etc/machine.secret
chmod 0600 /etc/machine.secret
If this does not work I am out of options or there might be some additional issue on your system.
/CV