Thanks for reporting this issue. I just opened a bug in our bug tracking database for tracking this issue. The cause of this is that the number of hidden fields in the page changes from the initial request and then subsequent request, and ASP.net Atlas just doesn't handle this really well right now. Specifically, the __LASTFOCUS hidden field only exists in the second request.
To workaround this, here are some possible options:
1. Don't use AutoPostBack in the DropDownList.
2. Use some control on the page that will trigger the focus hidden field to show up on the first request. I'm not really sure how to get this to happen though.
Thanks,
Eilon