Having same issue. Only hangs when loading thumbnails for the Materials shelf. All other shelfs load quickly.
Looks like it has trouble specifically generating thumbnails of custom materials created in Designer, although some (maybe 10%?) of my custom materials did generate.
Exported log file attached.
Edit: As a workaround, just to get my workflow up and running again, I found it easiest to take all of my custom materials out of their various locations (some in the Painter material shelf folder, some in a custom folder location) and move them to a new folder (thereby removing them from the Shelf when Substance loads). I can then reimport them individually, as needed, and that seems to work okay for now. This obviously is not a fix or long-term solution, especially for developers with large material libraries, but might help if you really need to get painting again.