Invitations to team blogs and private blogs are currently returning error code bX-tgu0qs. We're investigating.
Update: This error code is also appearing when logging in via the comment form.
Update (16:30 PST): This should mostly be fixed, though there are some lingering issues with Internet Explorer 6, which we're still working on. If that affects you, you should still be able to use these features from another browser, such as Firefox.