._new-cfg_kolab-vhosts-mailman.include.template and ._old-cfg_kolab-vhosts-mailman.include.template are being sourced by kolabconfig. This behavior should be changed. https://intevation.de/roundup/kolab/issue2074
With wrobel's retirement the only dev working on the kolab-overlay has left. WONTFIX seems to be the solution for the bugs until someone steps up.