When problems are unclear in the Samba logs, logging can be increased in the following ways:
in the smb.conf file, by editing (or adding) a log level = x line. This must go in the [global] section. Watch out that the logs might be become very large!
for a specific pid, by running smbcontrol <pid> debug x. This is the best for debugging, because the log level is "forgotten" when smb is restarted.
Quantum