/usr/share/icinga2/classicui/jquery-ui-addon is in icinga2-classicui 2.6.0-2+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/icinga2/classicui/../../icinga/htdocs/jquery-ui-addon
/usr/share/icinga2/classicui/../../icinga/htdocs/jquery-ui-addon