Frustrating.

It is possible your Firewall is blocking things. Before disabling though, I would try the following.

Record your v16 serial number and your purchase email address.
Cold start your machine.
Ensure no other Xara product is presently running.
Download the update from https://www.xara.com/designer-pro/download/, avoiding the Magix one.
Right-click on it and use the Run as administrator option.
If successful, register with your recorded credentials.

If this fails, come back and advise.

Acorn