samba - Jun 2008 - "Access denied" while trying to edit a group in usrmgr.exe

First of all, I am new to Samba, so please pardon me if I ask something
stupid. Thanks!

I recently set up a PDC using Samba version 3.0.28a. According to the
official Samba documentation, I should be able to use the Microsoft User
Manager tool to manage my Samba domain controller. I am able to
add/delete/modify user accounts with no problem, but editing groups is not
possible for some reason. For example, if I try to add a user account to a
group, I get an "Access denied" error message. This sounds a bit
strange to
me, since I log in to the domain as root, so privilege problems should not
happen.

Is this a bug or have I misconfigured something?

What I have already done:

- Install Samba from package
- Edit smb.conf to suit my needs
- Create basic group mapping with the correct RIDs (512 for domain admins,
513 for users, 514 for guests)
- Create a separated directory structure for all the shares

My shares are located on separate partitions, each have the user_xattr
option enabled in /etc/fstab.

I attached my smb.conf file to this message, to make it easier to understand
my configuration.

Thanks for you help in advance!