Error using Login Control with UpdatePanel

[Bob Ross]

I have an updatepanel with a little animation in it and I want to trigger this when the user clicks the button in the loign control.
But everytime I add a trigger of LoginControl - OnLoggingIn it throws this error the first time the site is run (after a service start or a long period)

Sys.WebForms.PageRequestManagerParserErrorException: The message recived from the server could not be parsed. 
Common causes for this error are the response is modified by calls to Response.Write(), reponse filters, HttpModules, or server trace is enabled.
Details: Error parsing near '_WS/SE/Welcome.aspx|<html>
	<head>
'.

Anyone know why this is? Or who to help?

[acoder]

This seems like an ASP.NET problem. Moved to the .NET forum.