I found the solution (3 steps)!
I applied the advice given here:
then I got another error (auth.log):
In /etc/pam.d/common-auth and /etc/pam.d/common-password comment out the lines that have pam_smbpass.so in them.
then I applied the solution posted on launchpad: (see messages #25, #26 and #27 - Vincent DAVY (vincentdavy) wrote on 2013-05-01)
May 10 10:22:16 ubuntu vsftpd: PAM audit_log_acct_message() failed: Operation not permitted
and then I got another error when connecting to FTP:
Download the patched version of vsftpd:
sudo apt-get remove vsftpd
sudo dpkg -i vsftpd_3.0.2-1ubuntu1_amd64_patched.deb
so I had to apply the solution posted here:
vsftpd: refusing to run with writable root inside chroot()
add thee following line to /etc/vsftpd.conf: