The AA slider is only adjustable for subscribed accounts. Free-to-play accounts have it locked at 50% if I remember correctly. If you are subscribed and the slider cannot be moved, then you have an issue.
There is also a bug in the default window where the number appears at 0 when you open the window, and does not display the correct value until you move the slider to change it. This is a bug in the default UI.
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