How get past invalid postback when using AJAX ScriptManager and UpdatePanel controls?

I'm using AJAX controls ScriptManager and UpdatePanel for the first time.

as I want a webpage that contains an embedded Windows Media Player to not be
interrupted as other clicks and events happen on the page (I have changing
content in an IFrame).

I put an ASP button on the page and click it but I get an "error on page".
In the details it says more or less the below:

Sys.WebForms.PageRequestManagerServerErrorException: Invalid postback or
callback argument. Event validation is enabled using enableEventValidation="true"/> in configuration or <%@ Page
EnableEventValidation="true" %> in a page... use
ClientScriptManager.RegisterForEventValidation method...

How do I get past this?

Thanks for any info,
Ron