Have you checked Database Agent compatibility? It is possible that the LN version is newer than the versions supported by your outdated NBU version.
The latest version in the 4.5 Agent matrix is listed as R5.0.10 on Windows 2000.
Has this worked in the past? If so, what has changed? Possibly LN and/or O/S upgrade?
One place to see what is happening with the LN backup on the client, is bpbkar log (increase logging level to a minimum of 3).