After moving to the new Adobe version I lost my old shelf. I've added the old shelf to the assets list, and it is in my assets list. When I open it in the assets window I only see some strange icons (see image). What's going on? The files are all still in the old shelf folders. I've even copied the starter assets to my old shelf and even they don't display.
These are LUT textures, they should not be placed in the materials sub-folder in your Assets folder.
You should also not touch starter assets.
All your custom assets should be placed in C:\Users\...\Documents\Adobe\Adobe Substance 3D Painter\assets with the same hierarchy as what you had in the olde shelf.
I prefer to use a different location for any assets I download or make, which is on a separate drive to the default.
I carried this forward from my Allegorithmic Painter installation to my Adobe Substance installation by just adding the top level "shelf" locationr to the library locations in Edit Settings. I have left the starter assets where they were installed by Adobe. All show up in the application shelf if I use path "All libraries".