The issue has been fixed. You can access your AdSense account by visiting www.google.com/adsense
We're aware of login issues some of our publishers are experiencing, and we're working to resolve the issue as soon as possible.
For now, you can access AdSense by logging into another Google account, e.g. Gmail first, and then going to your AdSense account.
We'll update you as soon as the issue is resolved.