Please take a look at post #4 in the thread below, and see if these checks will help you to get logged in.
[webmasterworld.com...]
AWA
Also, you may want to clear your cache. You may be trying to login at a cached page that has gone 'stale' somehow.
And, if you are logging in on a page that you got to from a bookmark, try opening a fresh browser and going direct to adwords.google.com for your login page.
Hope this'll get you in.
AWA