Samba 4 - Error / config changes

The following keywords should bring you here:

"gid_to_sid: winbind failed to find a sid for gid"
"for mask 0x2 returned (0x2)"
"Security token: (NULL)"
"force user" "force group"
"check_parent_access: access check on directory . for path anything for mask 0x2 returned (0x2) NT_STATUS_ACCESS_DENIED"

In samba 4 the force user has to be listed in the valid user list and has to have an entry in the password database.
"valid users = root, foo, bar
has to include the user in
force user = root"