I had this issue as well here in Canada with new internet. Was fine the first week but when Vault came out BAM, numerous network error codes. I called my ISP and they said they don't block any ports, yet when I went into my router and opened them manually I have no more error codes and an OPEN NAT. No idea what happened to Bungie servers, but something defs is going on with them but they keep blaming us.
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I have the errors even with open NAT. Opening ports is not an option for me as I have two computers playing the game, so I use UPNP instead of manually/fixed open ports.
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They say not to use both UPNP and Port Forwarding and I am. That fixed my issues but not saying it will for everyone. Something is definitely wrong with something on their end for people to be having so many issues. Even I had flawless internet before VOG. Then my NAT wouldn't get out of MODERATE and was getting constant cabbage, spinach, centipede, anteater, and weasel. So I have no idea what they did, but something definitely degraded on their end.