If I remember correctly the Sonys (maybe just older models?) have a filesize limit where a every single flow (text file inside the .epub, not the whole .epub) must be below 100 KB compressed and 300 KB uncompressed. The files in the Divine Comedy are above 300 KB, so you'll probably have to split them to work in your reader. I haven't used Sigil, but I'm sure it's quite easy to split the files there (look for "insert chapter break" or similar).
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