There is a new, confirmed bug in Google AdWords that will prevent some people from adding their billing information to their account.
AdWordsPro Bindu, an official Google representative, posted the details in a Google AdWords Help thread. Bindu said:
There is a known technical issue where some advertisers are unable to enter billing information and are redirected to the AdWords homepage.
Google has documented it in the known issues section:
We are currently aware of a technical issue due to which some users are unable to enter billing information into their accounts. Our engineers are currently working to resolve this issue.
There is no estimated time for a fix, but Google is working on it.
Forum discussion at Google AdWords Help.