A WebmasterWorld thread reports that advertisers are having to login twice to the Google AdWords console to gain access. I have confirmed the issue on my Mac running Firefox.
In short, you go to adwords.google.com and type in your username and password. Then you click login and it takes you back to the login screen. You then enter in the information in again and it let's you in. Second time a charm? Nah. It seems to be a cookie bug of some sort.
One user said Google is aware of the issue and recommended the user switch to a different browser. But the user said that Google said it was an Internet Explorer issue, which is not the case.
In any event, member RhinoFish suggested:
go to this url: https://adwords.google.com/select/
And try and login (don't use your password manager if you use one).
If it lets you in the first time, check your password manager (or bookmark) that you've been using and see what the url is that you have been using. if the url above works, rebookmark or edit the url in your password manager.
Forum discussion at WebmasterWorld.