Can't edit materials in one particular project

I noticed yesterday that I can't edit material properties in one of my projects any longer. I click on the spherical graphic on the left side of the material entry in the Inspector, but nothing happens. In other projects I can click on this and the property fields will pop down, but in this one project this has no effect.

Does anybody know of something that can be corrupted in a project to cause this to happen? I'm at a loss at what to do now. I tried doing a "re-import all" but it had no effect, and I'm out of ideas. I might have no recourse but to attempt to reassemble the project from scratch, and I'm not even sure how to go about that.

 

OK, I did finally fix this, but I can't say that I really pinpointed the source of the problem. I will post what I did figure out so that if someone finds this thread popping up in a search they will at least get the information I was able to glean.

I cloned the project folder and then started to delete stuff to see if I could pare it down to a point where material editing started to work again.

I finally got to the point where I deleted the Library folder, and when I opened the project again, after waiting awhile for Unity to rebuild the library, I found material editing finally worked.

I then cloned the original project again and then deleted only the Library folder, and again the resulting project had the material editing bug corrected.

I didn't give up there; I tried to determine if there was a particular file that triggered the bug. So I cloned the original once again and deleted files in the Library folder one by one.

I didn't have success until I deleted the ScriptAssemblies folder, then the material editing was enabled.

I thought that was the smoking gun, but then I went back to the original and deleted just the ScriptAssemblies folder, but that had no effect.

So, OK, it was ScriptAssemblies in combination with some other file/files in the Library folder. At this point I decided life was too short to waste any more time trying to narrow down this problem, so I just deleted the whole Library folder in the original project. I still have a backup in case there is a problem further on, but it looks like if you delete the Library folder it can be rebuilt with no apparent loss.