It is probably more of a Aeria Games bug than an Amber games bug....
After choosing 1100 AD from the Aeria site, if you left click on the "play now" button, with the Aeria Frame in place, the game will start normally... and then frequently revert back to the Amber Games log in. Since Aeria generated the password to the Amber games log in, this can cause confusion and frustration.
Work Around- If you right click on the "Play Now" button, and then choose either the "open in new tab" or "open in new window" options, you can play with out the Aeria framework. In most of the Aeria browser games, this will immediately start the game in that new tab/window. For this one, you have to click "Play Now" in the new tab/window.