Problems adding Audio Books to the Apple Books app

I've been using Books ever since it was released, and have a large collection of Audio books that I've collected over the years. Several months ago I suddenly had issues where I was unable to add new titles to the Books app. I tried 'Import' as well as dropping the file on the opened app and always got the 'failed to import' message. Backed up my computer, reformatted and reinstalled the current OS and all worked fine BUT as soon as a reinstalled my backed up files, importing again failed.

Does anybody know of a program or file (mostly text or pdf) that has the ability to stop files from being imported to Apple's Book app? It's totally frustrating me, though I have found a workaround - I now use another app to store my audio books so I can listen to them my iPhone when traveling.