Just wanted to add my own iteration of the problem several years down the road. I’m in CC2020 and I kept getting the same error when exporting to EPUB. I checked all my x-refs and didn’t have any problems. My problem was that I (like David) had pasted some text from a different InDesign document. This text had a hyperlink to a URL and got pasted into the new document as a hyperlink to a “Shared Destination” based on the other document I had pasted in from. I could see this when I selected the hyperlink and clicked “Edit Hyperlink”. I edited the hyperlink so that its destination was simply a URL and NOT a Shared Destination. Problem solved! So, if you can’t figure out the problem, you might also want to check your Hyperlinks and make sure they are only linked to Destinations within your document, or to simple web URLs, not Shared Destinations in other InDesign docs.