I have the exact same problem. All my material instances are showing this:
The materials themselves still work fine in the project, I’m just not able to edit instances, which pretty much means I’m stuck.
The only strange thing that happened that MAY be related is that shortly before this problem popped up, Unreal was unable to save two files in the ExternalActors folder. I couldn’t fix it in any way, so had to restart the editor. I noticed the material instance issue not long after.