I purchased and downloaded an EPUB using Adobe Digital Editions.
When I try Add to Library, I get the red exclamation mark in the notifications, with the message Error while adding: <book-name>.epub
I can see other downloaded EPUBs in the library, but I believe they were not purchased.
Anyone has any clue on what this error message could mean? Or how I could find more about it?

The full error message says:
Error while adding: /sdcard/Digital Editions/book_name.epub does not exist

If I try the Open in Reader option, nothing happens.