I think the DNS service in your ClearOS is not configured properly to comply thé requirements of Kerberos.
Have a look on the following page : https://wiki.samba.org/index.php/Samba_AD_DC_HOWTO
On your omv do the tests described in 7.3 testing DNS
I assume they will fail because samba 3 is not an AD. If I'm right you should consider using an other distro with samba 4. I'm sure about your needs, but based on what I saw on the homepage of ClearOS, zenthyal may be an alternative, if this distro switched to samba 4 (I tested it years ago).
An other alternative would be to setup yourself all the softwares you need ( the way I mostly choose ). I may share some interesting stuff.