[Framers] Strange error when updating Frame document

[David Spreadbury]

I recently upgraded to the latest FrameMaker2015 patch.Suddenly I am getting a strange unexplained error message:
'Book is not self-consistent'
Our documents are all in Unstructured Frame but I was running in Structured mode when I first received this message.I changed to Unstructured mode, thinking that might be the cause.I still get the message whenever I Update a book.
Does anyone know what this message means?
Thank youDave Spreadbury