Have you ever by accident set the CUPS spool directory to the same location ( RequestRoot /var/spool/samba/ in cupsd.conf or the other way round: /var/spool/cups/ is set as path> in the [printers] section)? These must be different. Set RequestRoot /var/spool/cups/ in cupsd.conf and path = /var/spool/samba in the [printers] section of smb.conf. Otherwise, cupsd will sanitize permissions to its spool directory with each restart and printing will not work reliably.