I have recently moved from PSE14 to PSE2020. I can see all my pictures (catalog of 12500 pics) but the miniature preview shows just about 50% of the photos. When I scroll thru the pics quickly the miniatures are there for half a second but then being replaced by he broken pic symbol immediately. In the information line at the bottom of the window I have a continouosly rotating symbol .. for more than 20 hours now. My computer should be quite capable (6 core,12 threads, 16GB RAM).
What OS are you using? When you say "the miniature preview shows just about 50% of the photos", I assume you mean that you can see 50% of the photo previews, and the other 50% show a broken picture symbol. Is there any pattern to which are shown properly and which aren't? Are any photos raw images? What is the source of the broken photos? If they come from a phone app, like Whats App, there is a known issue with their compatibility with the Organizer. If you click on a broken pic preview and press Ctrl+Shift+U, does the thumbnail update?
The (estimated) 50% I do not see show an hour-glass symbol (not a broken pic symbol.. sorry! my bad English, I'm German). The only pattern I see is that picture miniatures of the last three years are rather complete, older ones are (more or less 50%) invisible and have the symbol named.
I do not use raw images, all of the images are jpg (and a very few (~10) movies (mp4)). Also less than 20 are phone (Android) pictures.
Pressing Ctrl+Shift+U creates a question mark symbol for ~1 second which immediately changes back to the hour-glass symbol.