I'm having exactly the same problem right now. Called AE, and was told that United has a $xxx amount authorization on the card. I bought ticket from United.com and flown 2 weeks ago. AE told me to call United. I said how? I don't have United accounting dept number, and the regular folks in the reservation desks are not going to be able to do anything.

And because of this, I cannot purchase another big item on my AE. Very frustrated.