This problem occurs when your browser settings are restricting 3rd party cookies.
Safari on Apple iPad and Chrome Google Android tablets restrict 3rd party cookies by default.
Currently, the only way to avoid this error is to change your browser settings to never block cookies while viewing a lesson.
To do this on an Apple iPad, go to Settings -> Safari -> Privacy & Security -> then, change the "Block Cookies" option to "Never".
You should change this setting back to the default when you are done viewing a lesson.