ISPConfig3 + mailman + ubuntu 14.04 + CGI Group mismatch error

Hi,
I have ispconfig3 (now 3.1.6) running since years. All works well... but I recently added mailman to the environment.
Running mailman outside of ISPConfig seems to be more than a headache than there is progress. After moving '/usr/lib/cgi-bin/mailman' to '/var/www' I now get

Code:

Group mismatch error.  Mailman expected the CGI
wrapper script to be executed as group "www-data", but
the system's web server executed the CGI script as
group "client2".  Try tweaking the web server to run the
script as group "www-data", or re-run configure,
providing the command line option `--with-cgi-gid=client2'.

Whilst I do see 'Mailling List' in ISPconfig3 under 'email' I cannot administer a list I created there, ie I cannot add members, change lit settings etc. It appears to be detached or like a mock up part of the UI.
Can you please tell me or point me in the right direction, how to integrate mailman into ISPconfig post installation without screwing up the current ISPconfig configuration nor the existing mailing list, please?

Is there a perfect server how to for Ubuntu 14.04 + Apache2 + Mailman2 + ISPconfig?

Cheers,
Stefan

 

Hi Till,

Indeed, that has made quite a big difference as I can see the web UI of mailman now.
Thank you!
Is there any way to have one mailman instance and the different website accessing the WebUI?

Trying to administer the list make me loop at the form's post @ /mailman/admin/list/general : Whenever I try to apply a change, I get thrown back to the list's password prompt and upon entering none of the changes are safed.
Have you got any idea why that could be the case?

Cheers,
Stefan