Problem:
You have to change the AD Certificate. Therefore this certificate has to be changed for the AD connector.
Solution:
copy the certificate to /etc/univention/connector/ad
cp /root/ad_cert-20220301.cer /etc/univention/connector/ad/
Register the new certificate via ucr variable:
ucr set connector/ad/ldap/certificate='/etc/univention/connector/ad/ad_cert-20220301.cer'