Product Library connection to USGS:
Mission = Landsat8
Dataset = LANDSAT_8_C1 = Landsat Collection 1 (might be a problem, it should now be Collection 2)
Just select 2 dates and select an area
It first says: Searching product list 0 out of 7 (depends on your date and area)
Then it says: Failed to retrieve the product list from USGS.
I have filled out my credentials in the Tools, Options, then the Product Library tab.
I am using Windows 10 and SNAP 8.0.9.
Product Library connection to SciHub works OK.