[Samba] usrmgr.exe "group name could not be found" error

[Tom]

I've been having this problem on one of my PDC's where when using usrmgr 
from a windows box, I get the error: The group name could not be found. 
Since all of the groups samba is interested in exist and since they are 
all mapped to the appropriate posix groups I'm not sure where to start 
with this one.

A 'net rpc group list' gives the following:

Domain Guests
Domain Admins
Domain Users

A 'net groupmap list' provides the following:

System Operators (S-1-5-32-549) -> -1
Replicators (S-1-5-32-552) -> -1
Guests (S-1-5-32-546) -> -1
Domain Guests (S-1-5-21-1579542642-1944443579-1944909670-514) -> nobody
Domain Admins (S-1-5-21-1579542642-1944443579-1944909670-512) -> root
Power Users (S-1-5-32-547) -> -1
Print Operators (S-1-5-32-550) -> -1
Administrators (S-1-5-32-544) -> -1
Account Operators (S-1-5-32-548) -> -1
Domain Users (S-1-5-21-1579542642-1944443579-1944909670-513) -> users
Backup Operators (S-1-5-32-551) -> -1
Users (S-1-5-32-545) -> -1

smb.conf relevant parts:

passdb backend = tdbsam
add user script = /usr/sbin/useradd -m '%u'
delete user script = /usr/sbin/userdel -r '%u'
add group script = /usr/sbin/groupadd '%g'
delete group script = /usr/sbin/groupdel '%g'
add user to group script = /usr/bin/gpasswd -a '%u' '%g'
delete user from group script = /usr/bin/gpasswd -d '%u' '%g'
add machine script = /usr/sbin/useradd -s /bin/false -d /tmp '%u'
shutdown script = /var/lib/samba/scripts/shutdown.sh
abort shutdown script = /sbin/shutdown -c
map acl inherit = Yes

What log level should I look at if required?

This is running on Slackware 9.1, Samba 3.0.21b.

Thanks.