One big problem with Blogger, right now, is the confusion over authentication. Under classic Blogger, you could have a Blogger account, with an account name. That account could be tied to any email account, with any email service. Blogger Beta ties your Blogger access to a Google email address.
Unfortunately, even though you're not using Blogger Beta, you may be affected by changes to Classical Blogger, which support the Beta.
Blogger Employee seems to consider this possibility. And Blogger Buzz: Beta Update! describes issues that are relevant here too.
One reader writes of using the same account and password, in both Blogger and Google, and experiencing this problem. He reports a successful workaround, achieved by changing the password on one account.