This is the problem: after creating the book file, we had to move all the files (and the .indb file) to another server. When we open the .indb file, we get a red "?" stating the files cannot be found (but they are all located in the same (moved) folder). Has anyone an idea how the solve this problem without doing it manually (which works fine for a little book, but with some 90 files/book it becomes "a long way to go").
Move the folder containing the book and doc files so that they stay in the same position, relative to each other, instead of just the files. Otherwise, move them via Package for Print from the book panel menu in the top right corner, and target the new destination.
That's the problem, we cannot move all the files in the folder, just the ones used for that book. Our workflow works that way, as long as the files are not "print ready" they are in folder X. Once "print ready" they are moved (automatically) to folder B, but by that time the book file is already made.
Then the problem, unfortunately, lies in your workflow. You can’t just move some of the files. You’ve already found out why.