When there own repair tool corrects the issue.
I have done it 3 times on 3 different pcs with 3 different AV products (norton, mcafee and avg). Other people have also posted saying wow repair worked.
Yet not one post by a blue mentioning to try this before disabling AV software. Nor is anything mentioned that people should not have browser windows open or be browsing the web all willy nilly with no protection while wow updates. Just hey do this to disable that and then you can play....
Repair took all of 5 mins to complete and I did not have to disable any protection at all.