изменил конфиг:
location /video/sf/ {
root //192.168.xxx.xx/;
index index.html index.htm;

теперь в логе правильный путь
2015/07/29 15:31:56 [error] 11096#10252: *21 "//192.168.xxx.xx/video/sf/index.html" is forbidden (5: Access is denied), client: 93.126.68.47, server: localhost, request: "GET /video/sf/ HTTP/1.1", host: "www.site.com"

но не работает