I figured out that in some case the update panel will create runtime errors which happens as popup in the browser window with the error message "unknown error". I guess the same guy who have invented the hresult have created this feedback modell.
scenario
Using a ATLAS updatepanel and user can interact eg with textbox
Problem 1) user is typing reserved chars like < or &
Solution: set ValidateRequest="false" in page direktive
Problem 2) Page title or meta tags contains reserved chars like < or &
Solution: remove this from page title or comment out metatags in whole HEAD region
if you have additions pleas write it into comments