NT_STATUS_UNSUCCESSFUL is a common error. It usually means that the smbclient doesn't have a clue what happened... it just failed.

One way to find out is set the log level to 10 on the machine that is being connected to. This produces a lot of output so don't leave it that way.

Then, check the logs. On the machine being connected to, that's where the authentication happens. The log is usually the machine name that is connecting. So, if machine foo is connecting to machine bar then the log on machine bar which will be named foo is where to look.

This is a winbind issue. Winbind does all the authenticating for samba in the PDC paradyme. So if you are seeing this error... it's more than likely a winbind issue.

Run wbinfo -u. If that returns nothing, it isn't connecting to the PDC at all. Check Samba's config file. Make sure the password server is set. I think in the AD world this may not be needed... but in the PDC world it is.