Just recently this error (normally preceded by another error "Can't import books") has been occurring due to a compatibility issue with the latest update of Android 9.0 (Pie) and Ebook Reader 22.214.171.124. Please update the Ebook Reader app to the recently released newer version and this should resolve the issue. (You may need to uninstall and reinstall the app)
If this doesn't work please contact our support team. Please quote the error message, state which ebook (or ebooks) in your account you are unable to download and please also state your operating system version number and Ebook Reader app version number.