centos 6.6 - user dkim-milter does not exist

Hi there, I have a problem using dkim on my new fresh centos 6.6 installation. I use virtualmin/webmin and when i am going to “Email Messages” -> “DomainKeyIdentifiedMail” i try for the first time to save a dkim record, by changing the name to 2015 and checking “Signing of outgoing mail enabled?” to YES. As I click the save button, I see the message I have below: http://pastebin.com/xeX4QuZX

The user dkim-milter does not exist as I double-checked on my system by running “cat /etc/passwd”. I reinstalled dkim-milter and I receive the same error. I have another server with same configuration and its worked there from the start. I am not able to reinstall the system.

dkim version is the "dkim-milter-2.8.3-8.el6.x86_64

I also tried to use this tutorial to install it again: http://tecadmin.net/setup-domainkeys-dkim-on-postfix-centos-rhel/

but when I am at the end of step 3 and try to start dkim-milter I get again in the console:

Starting DomainKeys Identified Mail Milter (dkim-filter): runuser: user dkim-milter does not exist
                                                           [FAILED]```
I also find out that I was unable to create a user "test123" via ssh terminal, as I get:
```/usr/sbin/adduser test123
adduser: cannot lock /etc/passwd; try again later.```

my permissions: 
-rw-r--r-- 1 root root 2871 Mar  6 06:39 /etc/passwd

Why this is happening?


How can I fix this problem? I cant seem to find a similar problem on the net.