No parent page thumbnails in the pagepanel in Indesign 20.1

True. Most of the time, problems with missing parts of the interface mean the cache files are corrupt.

For the sake of other readers with a similar problem, here is a troubleshooting compendium.

I solved this problem by removing the cachefiles of Indesign. 

Go to this folder and remove it:  Bibliotheek/Caches/Adobe InDesign
Restart Indesign and the tumbnails of the parent-pages are back in the page-panel.