Good afternoon Ulf,
The command below will help you to reset the password of the domain master computer account :
udm computers/domaincontroller_master modify --dn "$(ucr get ldap/hostdn)" --set password="NewPassword"
- You then have to write it into the /etc/machine.secret.
echo -n 'NewPassword' > /etc/machine.secret
Maybe the article below will be helpful.
https://help.univention.com/t/manually-trigger-server-password-change/6376
Regards
Anna Takang