I exported a book and checked that with preflight for missing linked images and correct printing resolutions. everything came back ok. however one picture was not correctly linked and exported as the gray placeholder usually seen when "fast display" is enabled.

Apparently, the resolution of the gray placeholder is high enough as not to trigger the "too low res"-preflight setting of the printer. They did flag a different picture, which i replaced, but this square was undetected.

The image was "corrected" by relinking it, even though the linked path was correct and it did not show up in the linkpanel as faulty/missing.

I can't recreate the issue, so i don't know why the picture (even though it was linked correctly) did not show up or trigger any preflight settings.