Vaadin 7 Communication Error on UI run as Portlet but not as Servlet

I have a simple Hello World UI which has a single button which does a when pressed.

We are running tomcat6 and Vignette/OpenText Portal 8.2 along with Vaadin 7.0.1.

When deployed as a Vaadin7 Servlet, it works fine.

When deployed as a Vaadin7 Portlet, though, we get the red error box with:

Communication problem
Take note of any unsaved data, and click here to continue.(SyntaxError) stack: SyntaxError: Unexpected token ) at bKb (0.js:3716:68) at wJb (0.js:4955:46) at Object.iLb [as Kc]
(0.js:5262:24186) at At (0.js:4488:137) at Object.Ut (0.js:5262:3609) at XMLHttpRequest.eval (0.js:3673:65) at Sh (0.js:3015:29) at Vh (0.js:4686:59) at XMLHttpRequest.eval [as onreadystatechange]
(0.js:3846:45): Unexpected token ) - Original JSON-text:

Fiddler capture reveals

HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
X-UA-Compatible: chrome=1
Cache-Control: no-cache
portlet.http-status-code: 404
X-Vignette-RespondedWith: AJAX
Date: Fri, 01 Mar 2013 19:22:23 GMT
Content-Length: 0