I haven't used that software but it sounds like a problem with network B's routing table. I would recheck the route table. Also if it is an ICMP type 3 code 3 you should be able to capture that with an analyzer. Ethereal can do that. Analyze the packet dump. Look to see which device is returning the ICMP and what the actual port number is. The format for an ICMP type 3 code 3 is:
(IP header To: Original Sender From: Device that discovered the error)
(ICMP Header Type 3 Code 3)
(Failed IP Header: Contains Original Target IP: and Original Source)
(8 bytes failed payload 1st 2 bytes source port, 2nd 2 bytes target port)
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