samba - May 2012 - 3.6.5, Windows 2008 R2, not_defined_in_RFC4178@please_ignore

Hi list,

Attempting to join a domain with Samba 3.6.5 (ADS mode).

When running net join, The DC's for the site are properly retrieved,
but then I get this :

Doing spnego session setup (blob length=136)
got OID=1.3.6.1.4.1.311.2.2.30
[..]
got principal=not_defined_in_RFC4178 at please_ignore
[..]
SPNEGO login failed: Logon failure
failed session setup with NT_STATUS_LOGON_FAILURE


Searching for "not_defined_in_RFC4178" I find bugs that seem to have
been fixed already.
Any hint on how to diagnose what might be wrong with my setup.

I have tried with older versions of Samba and I get the same result.
I guess the problem comes from our new 2008 R2 DC servers. (we had
2003 previously)

Best,

Alex