I figured it out !

[Alex]

Hi
For all you guys who might have had the same issue. I found the answer on
some site after long searching.
Just uncomment everything in /etc/samba/smbusers and add a samba user called
root with smbpasswd -a root, give it a password and then use this account to
join the domain - works great. But I swear my other account was also part of
the adm group but it seems to have to be root and nothing else in order to
join a domain.

Alex