Have you been unbundled at your local exchange (LLU'd) - if so your line is probably no longer BT's resposibility -it will be AOL's.
Just because AOL asked you to use the BT speedtester means nowt. I have had the same scenario from TalkTalk(who now own AOL).
If I try and use the BT tester I get exactly the same message.
The recommended tester by TalkTalk for their unbundled customers is Thinkbroadband:
Give it a try.