In the file interface/lib/classes/tform_base.inc.php there is a bug at line 1190, the ip_address index of the $sql_v6 table is not surrounded by quotes, it therefore passes for a non-existent variable and causes a warning as well as an incomplete display of the error.
Thanks for your report, this stupid typo will be fixed in the next release. Relevant issue in GitLab: https://git.ispconfig.org/ispconfig/ispconfig3/-/issues/6597