I have this exact same problem: same symptoms, same "works fine in KTorrent", same settings on Deluge. The only difference is that I am using a Motorola SurfBoard 6120 Broadband DOCSIS3 cable modem and a Netgear FVS318G Router [with VPN and Firewall integrated].
I really like Deluge-torrent as a client, but this is kind of a deal-breaker for me,
Please bump up the priority to investigate and resolve this -- it will SURELY cost Deluge-torrent [perhaps anu] potential or current users..... We don't want that to happen!
Thanks for a really fine BT client, everyone, and all the hard work that obviously goes into i!!