I've now got kolab_delegate to work for me, but I got sidetracked in kolab-webadmin.
In the user config there is a list of delegates, where I added the delegatee, but
the user never saw any shared data (right way is using roundcube).
So I propose to either just remove the list of delegate from the webadmin, or to
display a read-only list of the delegatees (maybe nice for the admin, but might
also have derailed me).
Once I found https://docs.kolab.org/webmail-user-guide/roundcubemail/_plugins/kolab_delegation/settings.html
delegation worked for me.