Ran across error 0x000006d1 when attempting to add Samba shared printers to Windows 8 systems. The following fixed the issue for me. This may also help for earlier versions of Windows but I've only run across the problem with some (not all) Windows 8 systems.
Basically add (or edit) the following key in the registry:
Then reboot (restarting the print spooler is probably enough but it's easier to have a client reboot the system)