Hi Bookworm Girl,
The other thread created by me was because I couldn't import the library books into the Sony Reader Library. It was working before but then I had the protected page error so I did something and then I couldn't get the library books into the Sony Reader Library which in turns can't get it on my device. But that is fixed now...my issue still remains with the "protected page" message.
Yes, I do still have the prs-600 and am using the Sony Reader Library. I actually got this reader from my sister recently. I deauthorize her account and reauthorize it to myself. I made sure my computer is authorized in the Sony Reader Library. (in the "about reader library", my email address is there) I had previously done a hard reset and then I had to authorized the Adobe DRM protected content, which I did under my Adobe ID. I haven't purchased a book in the Sony Reader Library yet. I'm having this issue with library books.
I did what you suggested, deleted all library books from my ereader and from my hard drive (I even went to empty my recycle bin). Then I went into my library website and downloaded only one format and still the same protected page message.