Hi!
I am having tha same problem with my MBP 15" 2018.
After installing Big Sur the red squares appear every time trying to quick look something, it doesn't matter what kind of file I'm trying, it just appears briefly as when it is "loading" the preview. I discovered that it has to do with the external monitor, or the use of the discrete GPU, as when I don't have it plugged the preview displays fine.
I tried NVRAM reset, no success.
I tried on Safe Boot and found some issues:
- The previews have an intermediate step when opening in which they freeze midway and look blurry, and some kind of weird transparent frame around them, after some seconds, they finally open, and the fan starts speeding up, and when trying to close the preview, it freezes, and sometimes even brings me to the log in screen again. When I connect the secondary monitor and try the same workaround it kind of works better than without it.
- Tried opening Safari and scrolling down was super laggy. When connecting the external monitor, it again got a bit better.
- The red squares were replaced by this laggy, weird versions of previews.
- Back from safe boot, the red squares are still there.
- In Activity Monitor the processes CVM Complier, Window server and QuickLookUIService (Finder) were peaking in both CPU and Memory use.
- Both starting in Safe boot and regularly, I noticed for a brief instant a pop up window with the yellow warning exclamation sign, but it is too brief to even try to take a screen shot, so I have no idea what is causing it or what it is saying.
Maybe this report helps to find out what the problem might be.
Any ideas to try out are welcome!