It had been introduced Feb 12th by a confusion of the renderers to use, and unfortunately our automated tests didn't catch it. I submitted a fix that will be in the next build.
Also, look at the demo DB on how to display a menu. the XSP.openMenu function has 2 parameters, and your code should be like:
<xp:eventHandler event="onclick" submit="false">
<xp:this.script><![CDATA[XSP.openMenu(thisEvent,#{javascript:getComponent('popupMenu1').getMenuCtor()})
]]></xp:this.script>
</xp:eventHandler>
But you need the fix. I hope we'll provide a new build early next week.