There is no clear repro steps for a lot of project (especially projects with lots of materials, or materials embedded in fbx):

• Clear Library folder
• Open the project
• Open the Search Window (Ctrl + K)
• type t:material
• Wait for the previews to update. Sometimes some preview do not load properly.
• If you resize the Search Window or close and reopen it the Preview will properly load

Note that you need to clear the Library folder because previews are cached once they are generated.