wptdriver - test completed but there were no successful results


I have the testrunner setup and successfully running tests with IE8 and IE9. But any tests for Chrome/Firefox fail with the above message, and with a tiny partial waterfall image and no screenshot.

I’m using the Europe (Ireland) EC2 AMIs which I believe are 32bit, so I don’t need the connectivity=LAN setting in locations.ini

What else could be causing the above message?