To allow access to the Tasks backend, set up a global ACI that allows access to members of an Administrators group as follows:
$ dsconfig set-access-control-handler-prop \
  --add 'global-aci:(target="ldap:///cn=tasks")(targetattr="*||+")
        (version 5.0; acl "Access to the tasks backend for administrators";
         allow (all) groupdn="ldap:///
         cn=admins,ou=groups,dc=example,dc=com";)'