The /etc/apache2/conf.d/mailman.conf from mailman-2.1.17-2.1.7 contains
old-style access definitions:

<Directory /usr/lib/mailman/cgi-bin>
        order allow,deny
        allow from all
</Directory>

With these ones a call to a related cgi-url results in an 

"AH01630: client denied by server configuration:
/usr/lib/mailman/cgi-bin/admin"

error.although mod_access_compat ist loaded. :(

Changing the definitions to 

<Directory /usr/lib/mailman/cgi-bin>
        Require all granted
</Directory>

seems to fix this problem. Used apache version is apache2-2.4.10-16.1.