/usr/share/icinga2/classicui/ssi is in icinga2-classicui 2.4.1-2ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/icinga2/classicui/../../icinga/htdocs/ssi
/usr/share/icinga2/classicui/../../icinga/htdocs/ssi