Old files when reopened only display grey boxes for picture content

Like @jmlevy, I suspect it is the display settings. The high-quality command actually lives in three different places. Which one did you check? It's possible to have Prefs and View menu set to high-quality, but the individual files set to fast display. 

@jmlevy is showing this in his French version—select the images and either right click > Display Performance > High Quality or Object menu > Display Performance > High Quality (the menu he is using is cropped off in his screen shot).

~Barb

Check if you did not select the images and apply localy the fast display setting. See my example below:

If a document is not behaving well then try 

File>Export 

Choose IDML

Open the IDML in InDesign
Save the file as a new InDesign file with a new name
See if the issue persists.