Samba Winbind User Resolution
Now resolved, this worked for me:
Cleared all Winbind caches and flushed
Remember to take a backup before deleting anything!
Stop the Winbind and Samba services:
service winbind stop service smb stop
Clear the Samba Net cache:
net cache flush
Delete the Winbind caches:
rm -f /var/lib/samba/*.tdb rm -f /var/lib/samba/group_mapping.ldb
Start the Samba and then Winbind services - Note: The order is important
service smb start service winbind start
Test it by trying to resolve a user.
net cache flush will do.
I had to delete
/var/cache/samba as well, in addition to malco's answer (on Debian and Ubuntu)