I started getting this error this morning, and after quite a bit of nit-picking time, fixed it. I don’t know if your problem is the same as mine, but here’s what was going on:
I am working on an ePub. From time to time I “save as” a new incremental version. I was working on version 18 and pasted in some text from version 17, which I didn’t realize had a text anchor in it. When I exported the version 18 ePub, I got the error. On close examination I found that the text anchor was still referring to version 17, which by that time had been moved. Edited the text anchor and problem solved. Hope this helps.