What can cause difficulties is the use of VPN connections and similar tools.

There are many applications that suggest higher security by anonymizing data and Internet connections. Prominent ones being the Tor network, NordVPN, TunnelBear, Freegate, and many more. This can also be a service his ISP provides. Any such software must be deactivated.

Also make sure not to use a proxy server. These also redirect traffic.

Firewalls can also be an issue. They must allow outgoing and incomming connections for SilverFast. This can be on the computer or an internet router.

If a available try using a different Internet connection can help (e.g. a mobile phone hotspot).

If connected by WiFi see if you can connect through a lan cable.