Port range is same although you use more than 2 channels.
I guess your env does not configured properly allowing more than 2 channels(2 streams, in NetBackup term).
Check following attributes.
- Maximum jobs per client
- Maximum concurrent write drives
- (if you use number of streams more than drives - Multiplexing)
Maximum streams per drive, and Media multiplexing(in schedule attribute)
If you can not get success changing these attributes, enable debug logs on server(bprd, bpbrm) and client(vnetd, bpcd, dbclient).
Then figure out which connection was failed.