UpdatePanel doing full postback when it should not!

I had an UpdatePanel with a Timer for AsyncPostBackTrigger. But although my ScriptManager had EnablePartialRendering="true", my page always got refreshed. The problem was a setting in my web.config. I removed this line:

<xhtmlConformance mode="Legacy"/>

This setting was something that probably I forgot to remove when migrating from asp.net 1.1 to asp.net 2.0.