I was getting the "Protected Page" message when I first got my 600 - and it was when I was too eager to play with the reader and hadn't charged it much. I wanted to see what the books looked like on the reader, but with only a 1-2 bars of battery, it would not display the ePubs and give me the "Protected Page" message. After it was fully charged, I had no problems with it.
I am not sure if that is the reason why it is happening to you, but I found that was the reason for me. Maybe this will be helpful to you