Comment 1 for bug 1054665

> After logging again a user belonging to that new group, launching epoptes
> leads to 'An error occured while connecting: 13: Permission non accordée.'

After every group change, you need to restart the epoptes service, which uses the group to create its listening socket in /var/run/epoptes/epoptes.socket:
  sudo service epoptes restart

So, first, check if the epoptes service is running when your computer starts (using a samba SOCKET_GROUP for epoptes):
  sudo service epoptes status
Then, try restarting epoptes:
  sudo service epoptes restart
And finally, try running epoptes:
  epoptes

If that way you're able to start epoptes, this means that the epoptes service starts before samba starts.
In that case, check for the workaround mentioned in comment #12 of this similar bug report with epoptes and Likewise Open (LDAP):
https://answers.launchpad.net/epoptes/+question/208472