Your portlet renders fine but when you try and use one of the controls, like changing the value of an ice:selectOneMenu control for instance, you get the dreaded session timeout.
As with so many of these issues, there seems to be an endless variety of solutions out there. In our case, we're talking specifically about a situation where Liferay is NOT installed as the ROOT context on your Tomcat.
To fix this, you need only go into your [Tomcat Home]/conf/server.xml file and find the relevant Connector setting. That's usually the one that says Connector port="8080" protocol="HTTP/1.1"...
Add the following attribute:
This is something of a workaround, but it is the accepted solution. For more info, click here.