I administer WSUS from day 1 (2002 or so) and I often import updates into WSUS using the GUI.
This worked until recently. Now it throws an error 80131509 ("some updates could not be imported"), no matter what update I choose and no matter on which WSUS server. i tried on server 2016 and on server 2012 R2, both fail. Administrators from the german forum administrator.de did confirm these findings:
Please investigate and correct that.