I got this icon for the first time today. The folder was created via a program that I am sure was not using Windows API, as such the folder was not part of the vault, it was local.
It looked like that only in the tree, looked like a normal gray folder in the folder list.
Once I added that folder to the vault, everything worked normally again.
I appreciate the post, just was curious as to what it was. It did appear like this only in the tree.