I suggest that -v option (verbose) would be added to smbclient. For example, I configured my smb.conf and testparm validated it as valid. However, smbclient -L says "protocol negotiation failed". I would like to know what exactly failed in what protocol negotiation, but smbclient doesn't have any -v switch.
use a nigher debug level.
to be more specific, use --debuglevel=[0 - 10] depending on how much you want to see.