I ran into a problem using Atlas on my page which resulted in a popup showing me an "Unknown Error" alert.
To fix this, try the following:
2. Next, ensure that you are not making any Response.Write(s) to the page.
3. Ensure that you have no HTML tags outside of your documents <html></html> tags.
4. Wrap all update-able controls in an Atlas Update Panel.
5. Finally if non of the above helps and you can afford to take the security risk, add the page directive EnableEventValidation="false".