No luck, I'm afraid. My error message, fwiw, is "Cannot download. The content is not supported on this device." I tried three works, and I rebooted my ereader in between.

Now, this is a new toy for me, but I have successfully downloaded one other epub file today, one not on the AO3.

I'll see about downloading the files onto my PC and running them through Calibre, as others have suggested. Not that that's any help to the coders.