This error occurs on some models of Sony Readers when transferring protected eBooks and alternating between PDF and EPUB formats.
To resolve this error, transfer a new book using the same format as the one giving you issues. After that, you should be able to open eBooks in the new format.
For example, if you transfer an EPUB eBook to your Sony Reader and later get the "Protected Page" error when attempting to open the book, you need to transfer another EPUB eBook to your device. This will clear the "Protected Page" error for all EPUB eBooks on your Sony Reader.
Sony has fixed this issue on the following Reader models:
If you have one of the above models and are still experiencing difficulty, please try updating the firmware on your device.
If you want to transfer the file to the reader via ADE but the EPUB file you have is not in Adobe Digital Editions (ADE) you can add it to your library using File > Add to Library
If you don't have any files on your computer to use, you can download a free ebook file from https://www.gutenberg.org/
If you require further assistance please contact our support team. Please quote which reader model you own and describe what steps you have taken so far and the issues you have faced.