The ball thumbnail in materials list sometime is empty and sometime remains grey
Hi Timor,
Maybe it'll update if you wait a bit. It can be a bit slow at times. (may also depend on which Blender Version you are using).
You can also try to Save the file, close Blender and Open it again.
If it's a persistent problem, make sure that your Graphics Driver is up to date.
You can try to switch from Vulkan to OpenGL in; Edit > Preferences > System > Display Graphics > Backend.
But most importantly, nobody uses that thing, its use and utility is long gone. I mean if it was useful at all I'd be worried, but we have so many other tools now that do ten times a better job. So don't worry about it, you can collapse that window and have it collapse for all eternity.
Now that you mention it... I feel like I've hardly ever really used the material preview area since 2.4x days. Or very early 2.6x. These days, I just look at the object in the viewport that I'm putting the material on.