As most people do, to load YUI I used:
which caused following error:
[ERROR] Failure to load module 'null'
com.google.gwt.dev.shell.HostedModeException: Can't find frame window for 143347568
but only when running in GWT’s hosted mode, so it looked like a bug in Linux implementation of hosted mode’s engine (most people use Windows and do not report such problems).
After some experiments I found first workaround: using JQuery instead of YUI. Full ExtJS setup looked like this:
<link rel="stylesheet" type="text/css" href="js/ext/resources/css/xtheme-aero.css" />
<link rel="stylesheet" type="text/css" href="js/ext/resources/css/ext-all.css"/>
finally solving the problem.