just happened by this post when i was searching for a fix for this problem. i dont know what happened to my samba install, but it borked on me and started spitting out this error.
to fix it, i had to remove samba from my system and completely reinstall. im pretty sure it had something to do with a corrupt smbpasswd entry. the first time i reinstalled, i got the same error. the second time, i backed up everything in /etc/samba and then deleted all the files in that dir, and reinstalled again. from that point, all was well again.