Apple - Can't open some ePub books in Books.app on macOS Mojave
If your ebook indeed is unencrypted, you can try to open the ePub with the free Sigil.app (an ePub creator/editor) and write another copy to disk.
Sigil is able to repair a lot of structural problems. You can get a precompiled dmg as well as the full source code here.
I agree the "social DRM" should not pose any obstacle for using these eBooks with Books.app. But I suspect the "patching personal data into the eBooks" is done by an automated process, which simply went awry in your case. Maybe you even have some special characters in your identifying information (quotes, accents, ampersand, general unicode characters).