This issue started occurring for me recently and I urgently need a resolution to the problem as it prevents generation of new books for product team reviews. Specifics:
* Until recently I was able to save and generate a book file. Changes were made to various .fm files that are part of the book and now when you attempt to update the book, this error is returned.
* I did a clean install of Framemaker on a second machine and the same error occurs for me.
* Other users appear to be able to open and regenerate the book without issue on their machines.
* Other books appear to generate correctly for me.
What version of FM are you running? Are you all patched up? Check Help > Updates to check. I would suggest MIF-washing the book that seems messed up.