Mine is doing it too. If I access the dictionary from while reading a book, I get the message "Activity Reader (in application Reader) is not responding". If I try opening the dictionary directly from Home, I get "The application Reader has stopped unexpectedly. Please try again."
Mine just started doing this as it was out of warranty. I has alos seen that Nook users are reporting the same exact message. No one seems to have a solution yet ( Besides buying a Kindle )
I have just posted it to the Sony support forum, so we'll see if Sony has an answer.