I too have had / am having the same problem. Mine occurs when I try to transfer scrapbook / template files from my work computer to my home office.
My thought process was to use a flash drive and download the template and the scrapbook I use most, and upload them to my home computer. Let’s say I uploaded them to my desktop:
When the files are first opened (from my desktop), all the images are referenced correctly & there are NO red X’s. However, as soon as I save it as a scrapbook, 1/3 of the images turn into a grey box with a red X through it. (But not all of them…?)
What’s even weirder is that if I open (1) a blank layout, (2) the LO-saved scrapbook, and (3) the desktop-saved scrapbook, I can drag the “Red-X” files from (2) into (1) and they will update the reference correctly. But if I close (3), they will still show the red X.
This makes me feel that LO wants to reference the initial LO file that was used to make the scrapbook, even though a new LO file is made in it’s place. What’s weird is the inconsistency of it since only 1/3 of the images get the X and the other 2/3 still work fine…
UPDATE: I still haven’t posted this but I actually just found a fix for my issues. Since my “desktop-saved” version was showing the images correctly, I manually moved that file into the Scrapbook save location at C:\Users\USERNAME\AppData\Roaming\SketchUp\SketchUp 2018\LayOut\ScrapbooksThis solved all red-X issues. So for me, it appears the LO save process is what was causing the errors?