Unfortunately that error shows only that pam_unix believes you failed to login correctly (i.e., that the password typed was wrong); enabling or disabling pam_smbpass would have no bearing on this.
I suppose it could have an effect on gdm's behavior /after/ a failed login, but that doesn't seem to be the problem you're reporting?
Are there no other PAM log messages at all from the time in question? Note that by this point, the auth.log has probably been rotated, so that the relevant log has now moved to /var/log/auth.log.1.gz or similar.
Unfortunately that error shows only that pam_unix believes you failed to login correctly (i.e., that the password typed was wrong); enabling or disabling pam_smbpass would have no bearing on this.
I suppose it could have an effect on gdm's behavior /after/ a failed login, but that doesn't seem to be the problem you're reporting?
Are there no other PAM log messages at all from the time in question? Note that by this point, the auth.log has probably been rotated, so that the relevant log has now moved to /var/log/ auth.log. 1.gz or similar.