I posted a thing about this on my 300. I have a 505 and a 600 too (as u can see, lol). I received my 600 for a bday pres in October and I don't have a problem reading lrx files on it (lookie Don, I got it right-yayyy!!!). I just bought my 300 last week and that message comes up when I try to move my books. Read the post on the 300 cuz someone mentions why it's happening where some can read them, some cannot.
https://www.mobileread.com/forums/showthread.php?t=67729
Hope this helps everyone!!!!
P.S. Watch out for blond moments in my post-my intentions are good but sometimes it takes a while for my brain to catch up!!!!