In this game: 150082
I attacked amazonas from north east brazil, and then tried to advance the remaining 2661 armies onto amazonas.
When I clicked 'advance' I got and error saying 'Illegal attack move!', and was unable to move the armies anywhere.
I tried this several times, and tried using IE as well as firefox, and even tried reducing the number of armies down to as low as 1800-ish, and still got the same error.
In the end I had to select 0 armies before it allowed me to progress.