Not using ubuntu, but this issue is happening to me and seemingly to a lot of 
folks.
I think I've found the cause.

In my case, it was a matter of incorrectly configured DNS causing 
creds.get_named_ccache()
to fail to find the KDC with which to obtain credentials. No core is being 
dumped in my case;
I was just getting this famous stack dump when trying to do:
    /usr/local/samba/sbin/samba_dnsupdate --verbose --all-names
as is suggested in the how-to I was reading to set up Samba.

A couple minutes with strace revealed that it was trying to look up 
vmnet-dc1.vmnet.us
using my upstream ISP's name server. At first I just put 127.0.0.1 in the 
/etc/krb.conf for
the kdc and admin_server settings in the [realms] section. This got me past the 
stacktrace
but I was stilling getting a lot of '. .. blah blah blah not found' errors. 
Then I realized that
I had left my ISP DNS in /etc/resolv.conf when I was wget-ing a needed source 
tarball, and
forgot to set it back to 127.0.0.1 (to use the samba-populated DNS server 
locally).
Herp derp derp! :) So I set resolv.conf to point at 127.0.0.1 and set krb5.conf 
to use the
correct DNS names for kdc, etc. Bingo! It now works perfectly.

Bottom line with this "bug" I think is that you need to be VERY careful to set 
up DNS
right or things will fail. (DNS tends to be that way)

/etc/resolv.conf:
------------ snip snip -------------------
search vmnet.us
nameserver 127.0.0.1
------------ snip snip -------------------

/etc/krb5.conf
------------ snip snip -------------------
[logging]
 default = FILE:/var/log/krb5libs.log
 kdc = FILE:/var/log/krb5kdc.log
 admin_server = FILE:/var/log/kadmind.log

[libdefaults]
 default_realm = VMNET.US
 dns_lookup_realm = false
 dns_lookup_kdc = true
 ticket_lifetime = 24h
 renew_lifetime = 7d
 forwardable = true

[realms]
 VMNET.US = {
  kdc = vmnet-dc1.vmnet.us
  admin_server = vmnet-dc1.vmnet.us
 }

[domain_realm]
 .vmnet.us = VMNET.US
 vmnet.us = VMNET.US
------------ snip snip -------------------

-- 
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Bugs, which is subscribed to Ubuntu.
https://bugs.launchpad.net/bugs/809638

Title:
  samba_dnsupdate crashed with RuntimeError in get_credentials(): kinit
  for INTEL$@REALM failed (Cannot contact any KDC for requested realm)

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