Why not do what the error message advises you to do?
Hmm, that is strange. I do have a couple ideas that may work, but I have never seen this error myself.
1. Try restarting the computer. I know, lame answer. But I'm thinking if the process the launched the other episodes gets hung up, then netflix will think you are currently watching one of the episodes you've already finished, and so won't let you start another one.
2. If that doesn't work, then make sure nothing has changed with your windows firewall, or antivirus software. When you watch something on netflix, it has to do a DRM check on your computer, so your computer sends information back to them. It sounds like somewhere along the way, this DRM response isn't getting back to their servers, so they won't stream the video.
Hopefully one of those works. It is strange that you were able to watch fine in the past, though, and now you can't (I'm assuming you haven't made any significant changes to your computer since then).
I called NetFlix and they took care of my problem! They switched me over to their new beta player, now everything works fine. Even the picture is clearer!
Netflix Begins Roll-Out of 2nd Generation Media Player for Instant Streaming on Windows PCs and Intel Macs!