I've had this issue since December, can't watch anything on Netflix on the V6 box. I believe it's some sort of corrupted cache issue as the suggestion from Netflix for other devices is to uninstall and re-install the Netflix app, however, this is not possible on the V6\TiVo.
It's possible a full reset of the box might fix it, but this means you'd lose all recordings, series links etc. and that's not a guaranteed fix (plus the problem could return).
I've basically given up on Netflix on the V6 for now and just use another device, which is a shame as I like the Series Link+ feature and I tended to watch a lot more on Netflix when it was available through the V6. Test every month or so to see if it's fixed, but no change yet.