Yesterday, we reported of wide spread Google AdSense login issues. It seems like after receiving more information from Google on the matter, it might be two separate issues.
One is a standard login issue for some accounts and the other is an issue with accounts that need to be migrated off their original email accounts.
The second issue is known as a "migration loop" issue and there is a Google AdSense Help thread specifically about that, where an AdSense representative shares how to fix the issue for now:
We're aware of an issue where many AdSense publishers are stuck in a Google Account migration loop when signing into AdSense. We've thoroughly researched the issue and wanted to share the details with you so that you can take the appropriate steps.
Publishers experiencing this particular issue (getting stuck in an Google Account migration loop) are those whose original AdSense email address *and* password match their existing Google Account email address and password. In other words, if you signed up for AdSense before we asked everyone to update to Google Accounts, and then you later created a Google Account with the *same* email address and password (and haven't changed the password since), then unfortunately you're encountering a bug in our Google Accounts migration code.
Fortunately, there's an immediate solution to this issue. If you're stuck in this Google Account migration loop, simply visit https://www.google.com/accounts, sign in, and then change your Google Account password to something different than what you're currently using. In addition to being a good password management practice, you'll eliminate the migration loop with this change.
We apologize for this issue and are actively working to put in a more permanent solution. Thanks for your patience as we work to resolve this issue.
There are many threads on the topic, and here are some of them.