Apply the corect size to the master pages so that it matches the Document Setup.
Apply the Master Page to all pages in the document.
I had already done that. I am still getiing the error message. Tried it again for good measure but still didn't work, thank you.
This is a known bug.
Copy/paste from the bad file to a new one
Copying all the elements from each page into a new document doesn't always work.
Following that advice, in a completely new document, a page that included image across the fold seemed to cause the error.
Pasting everything except that image prevented the error, but the result was not usable.
We had to manually impose printer spreads.