Occasionally material and material functions previews in the content browser are displayed with the default material checkerboard, perhaps due to shaders not being compiled.
UDN users are also experiencing it. Relative UDN case here https://udn.unrealengine.com/s/question/0D54z00009itjvcCAA/editor-always-freeze-when-saving-a-materialinstance
1. Open the attached MaterialFreeze project.
2. In the content browser, navigate to Content > Material Library > VFX
3. Observe that Material Functions are shown with the default material. Opening a material function will update the preview.
Expected: the real material preview is always displayed.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-200553 in the post.