I just got a new iMAC running Big Sur. I downloaded DropBox and it syncs fine. But QuickLook doesn't work. That is, if I click on a file to highlight it, and hit the spacebar, I get this kind of window:
But if I do the same thing on a file that is NOT in my DropBox folder, it works fine.
Another clue: if I try to preview the above file with QuickLook and get the above window, and then click on "Open with Preview", then QuickLook will work on that file even after I exit from Preview. So perhaps it has to do with Dropbox not bringing the preview (thumbnails?) over when it copied over all the files in the first place.
The problem is solved. Here’s what happened: when I installed Dropbox I had smart sync set to online. So it didn’t actually copy the files over. So no quickLook preview! When I opened the file locally, then it copied and then quick look previews worked as usual. The clue that I should have spotted earlier was that my hard drive was almost empty, but I had several hundred G of files!
So I set smart sync to local in the Dropbox preferences. However that only changes future files. I had to go to my Dropbox folder, view and select all the sub folders, right click and set smart sync to local. This is tricky - when I right click, it showed local but I had to click on local anyway to force copying everyone over. But that did it. Now all files are local and quickLook works fine.