I have the same problem and haven't found a solution, went through a phase of doing it in late November, on some shows (always the ones I wanted to watch obviously, if I picked something I didn't want to watch it would play fine), then got better for a few weeks, now seems to have stopped working for everything since shortly before Christmas.
I've heard that on other devices deleting and re-installing the app, or clearing cached data fixes it, but those don't seem to be options on the V6 as far as I can see.
I also searched on the error before posting here and performed the suggested Netflix troubleshooting which is only restarting the box (done) and checking cable and ports (which I have also done, replacing the cable with a premium new one and using alternate ports).
Neither fixed the issue and would not explain why it worked for 4 days recently after deleting the stored key.
The forum has a large number of these types of issues reported, it would appear to be quite a widespread issue.