RC4 Error message adding a user

[Adel]

hello
I first searched the forum about this problem and found a post but
i couldn't find the solution for it, this solutions didn't work on mine
i never had this error on RC3 , but i uninstalled RC3 and reboot then
installed RC4, I did that because when i tried to upgrade i kept getting
errors, so that was the only way to install RC4,this is happened when
i tried to add a new user:
sys_command() | ERROR: External command '/usr/sbin/groupadd -g 2006 vu2006' returned '9' status !
I don't know where is this coming from, i even deleted and put it back again
and no luck.
I have 2 IP address on ispcp : public IP for DNS & private IP for the server
this happen when i use the public IP address, which is the one i should use
for the domains DNS.
Any help will be very much appreciated.
Thanks