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.