The endpoints used by #WSUS for manually importing updates like the “Cumulative Update Preview for Windows 10” are not capable of TLS 1.2. So, after installing the TLS hardening via July updates on a WSUS server the import fails with error 80131509.
To reenable the import you need to allow the fallback to the less secure TLS versions.
Create a ‘w3wp.exe.config’ file in ‘C:\Windows\System32\inetsrv’ with:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
    <runtime>
        <AppContextSwitchOverrides value="Switch.System.Net.DontEnableSystemDefaultTlsVersions=false"/>
    </runtime>
</configuration>

And run ‘iisreset’.
Be aware that this allows the TLS fallback for all w3wp instances and is not limited to WSUS.