As a measure to help keep the database size more manageable, NTA allows you to monitor any combination of TCP and/or UDP ports. Any unmonitored TCP/UDP ports will show as Unknown, and have their port number rewritten as -1. "Port (-1)" should be appearing as "Unknown" - I don't know why it would be appearing with that port number. Hope this helps.
Is IP CEF running on the other router?
I think it is required for netflow to function. Should be on by default but you never know.