I'm having the same problem, in the United States. Netflix had been working fine on ATV2, and then suddenly doesn't. Its keeps saying the Netflix account or password is invalid. They aren't. Netflix continues to work fine on iPhone and MacBookPro using the same wifi and isp, also all in the United States.
The "activation code" people are refering to is what Netflix is asking for to "activate your device" when trying to work the problem through Netflix. My bluray player gives such an activation code. ATV2 apparently doesn't, so it's a dead-end trying to troubleshoot this stupid thing. Does anyone have any input other than, "you need to get on something called The Internets and activate something called a Netflix Account, then you simply log into that account and your problem will go away"? Because that's not helping.