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I have the exact same situation with my Sony bookstore epub books I tried to convert to .lrf after I made the font larger. I thought it was the newest Calibre so I went back to the older Calibre software and I got the same problem. Some of the books worked and some did not. They crashed the Sony ereader so I finally gave up! Apparently the book is corrupted. On another posting I listed the error I got. I finally decided I can't fix it, until some expert has the same problem and fixes it I am at a loss too. Keep me posted on how you fix it.

Oh yeah, I also unzipped and zipped the file again, as someone here said that helps when you can't open an .epub book. Apparently that was not the problem as it still did crash the Sony.