The sync code is your password for the sync chain. It stays the same so long as the sync chain exists. So if you had your first laptop that you created the sync chain and then you added your second laptop to an existing, providing the code your first one gave you, then it shouldn’t be having any issues.
When you select new device, that’s so it can determine which of the codes to give you. For mobile it will give you the QR code and for a computer it gives you the long phrase that you would put in. Both of them work the same way it just is different presentations.
Well gives me is usually it’s something very simple that gets missed that causes issues. But maybe yours really is some oddball situation here. And I’m sitting here trying to respond to everything awesome been wanting to get to sleep for a while. It’s a little after 2 AM here so I’m gonna have to come back to this later. For right now though I am gonna put this link below and say that maybe we can see if @Mattches has any ideas