Ucs5 join dns error

ucs5, client: Linux Mint 20.1 xfce

trying univention-domain-join-cli
in ‘/var/log/univention/domain-join-cli.log’ I got:

2021-05-29 22:34:14,064 userinfo INFO Created a backup of all configuration files, that will be modified at ‘/var/univention-backup/20210529203414_domain-join’.
2021-05-29 22:34:14,417 userinfo CRITICAL An error occurred: The UCS master name s1.xxx.intranet can not be resolved, please check your DNS settings. Please check /var/log/univention/domain-join-cli.log for more information.
2021-05-29 22:34:14,417 debugging CRITICAL The UCS master name s1.xxx.intranet can not be resolved, please check your DNS settings
Traceback (most recent call last):
File “/usr/sbin/univention-domain-join-cli”, line 189, in
distribution_joiner.join_domain()
File “/usr/lib/python3/dist-packages/univention_domain_join/distributions/linuxmint.py”, line 108, in join_domain
raise DcResolveException(‘The UCS master name %s can not be resolved, please check your DNS settings’ % self.ldap_master)
univention_domain_join.distributions.linuxmint.DcResolveException: The UCS master name s1.xxx.intranet can not be resolved, please check your DNS settings

Best regards, Peter