volgens Colin Fraser.
Could it be that the Book module went from a third party plugin to core Moodle? It was fairly recently this happened so that may be the reason. Do a backup of a 2.3.2 course with a book in it and extract the moodle.xml file, compare the addressing strings to the book between the 2.2.4 and the new, they should be the same, but perhaps not, that might give you a clue...