The folks at WebmasterWorld are frustrated with a relatively new implementation on the Google AdSense side where they are being asked to update their login information.
The message is as follows:
"Welcome! You're signed in to Google Accounts under the email [your email address] and your Google Account password, but this is not a valid AdSense login."
The reason for this, according to the Inside AdSense blog, is because your AdSense account shares the same login and password as a Google Account but the account is not linked to the AdSense account. When you log into www.google.com/adsense, you've gotten access to your Google Account but not your AdSense account. For now, to get around it, you need to maintain two separate passwords.
Most users are merging their accounts. Others don't want to bother. As one person puts it, though, "just do it." It doesn't take long and the frustration will go away.
Forum discussion continues at WebmasterWorld.