Problem resolved. Thanks. I had duplicate
<filter>
<filter-name>app</filter-name>
<filter-class>org.apache.tapestry5.TapestryFilter</filter-class>
</filter>
in my web.xml.
+1 for eclipse IDE. It did complain about duplicate in web.xml and so helped
to pinpoint the problem, while Netbeans (which I am usually using) remained
silent
