I have a user, who is unable to log into her account via Advanced Mode (AJAX). She's able to log in via standard without a hitch. This problem arose when we upgraded from 5.0.6 -> 5.0.9.
Here is what IE7 reports as the error when trying to login using Advanced Mode:
I used server instead of our DNS name. Trust me it's a legit url otherwise..
Error: '_field' is null or not an object
I have 150+ users and everyone is able to login using AJAX but her. I've tried logging in using the Admin UI and get the same error. Doesn't matter what computer I use, that account can't login using Advanced mode.