I had the same problem when using Google Drive and Dropbox, not sure what the issue was, I do not remember, something about those files creating temporary files whenever they were accessed, even from other apps and those files were getting stored in a temporary memory location. I have the same problems when trying to edit an image for over 30 minutes from the Cloud drives. Some such tech talk my IT person told me. Have you tried setting the images to a lower preview setting, that helped me about half the time. I wish I had better news, but I do not think that it is not a limitation in Scribus, InDesign also has problems with cloud drives (except adobes own) and file placement. In my opinion it is more an OS problem and how cloud drives interact with the desktop file system.
mnawij