Problem with YouTube on iPhone

I have an iPhone 4 and often when I have shut down the phone (that is, clicked the button on the top, not powered down the phone), I will get a message stating that I am not authorized to view the video I was previously viewing. If I click done, and try to open other videos, I have the same issue. If I go back to Search, the problem goes away. This is typically on WiFi. Any idea what’s going on here?