Can't transfer books to eReader

Suddenly in the past week, I can't transfer books, either DRM or DRM-free, from my computer to ADE. Have not had this problem in the past 3 years! I'm using ADE 3.0, but I've tried installing ADE 4.5. ADE 4.5 doesn't appear to be compatible with my library's Overdrive software. I have even tried deleting a few books on my eReader and re-installing them. They appear to transfer to the eReader, but when I try to open them, I get a blank page. I'm using a KOBO Aura reader, with current firmware. Also, the books I've bought from KOBO don't exhibit this problem. It's only when I try to transfer (import) a book from ADE to the eReader that this happens!!!!