I finally managed to finish Victory or Death. I disabled the firewall and ping blocking on my router. I still had the windows firewall on, with all ports open in/out for gw2.exe and out for coherentui_host.exe.
I also reinstalled the whole game and deleted local.dat earlier, but that didn't help for Victory or Death. Some time ago I had trouble doing the final Kralkatorrik mission, so I reinstalled and managed to finish the Kralkatorrik finale without touching the firewall settings. I don't know what's going on, but those are my success stories after trying over a dozen times.