I have also tried disabling the Pepper nonsense in Chrome.
Two things to try. I'd be curious about feedback on whether or not one of these works for you. This will work for the non-PPAPI browsers. PPAPI Chrome is it's own beast.
Open Settings/Control Panel > Flash Player
Under Local Storage, choose Delete All
If that doesn't work:
Under Advanced > Browser Storage and Settings, choose Delete All
Fantastic, it worked. I have no idea why, but I'm assuming there was some kind of license file I had downloaded that was preventing me from playing those videos. The Firefox error message in Google Play wanted me to reset my license files using the settings manager, but I guess it just wasn't doing what it was supposed to. Points for Jeromie.
Sweet, that's great to hear.
I googled for that error message, and what I gleaned based on reading a bunch of the threads was that it wasn't directly related to the DRM vouchers, but to a Local Shared Object (aka Flash Cookie) that was probably maintaining your login session. My guess is that the content providers had to restart their Flash Media Servers, and the LSOs associated with the content provider then referred to a session that no longer existed or something. Blowing away the LSOs seems to fix it. Given that blowing away the LSOs does resolve this problem, logging out of the content provider's service completely and then logging back in would probably get you a new session token, which would most likely also resolve instances of this error message.