I had the same problem, and I found that when I add the legend attribute placement, that is when the chart disappears. The legend shows with the placement indicated, but no chart.
I finally figured out what to do, I started from scratch with the bare minimum pie chart with examples from this page:
WebCharts3D Piecharts
And I discovered that the order of the elements in the XML file was the issue.
This is how my XML file is now and it works:
<?xml version="1.0" encoding="UTF-8"?>
<paint paint="Plain" />
<title>
<decoration style="None"/>
</title>
<dataLabels style="Pattern" font="Arial-16-bold">
<![CDATA[
$(value), $(colPercent)
]]>
</dataLabels>
<legend placement="Right" font="Arial-16">
<decoration style="None"/>
</legend>
<insets left="10" top="10" right="5" />
<elements action="" shape="Area" drawOutline="false">
<morph morph="Blur" /> <!-- other options: grow, none -->
<series index="0">
<paint color="#E48701" /> <!-- orange -->
</series>
<series index="1">
<paint color="#A5BC4E" /> <!-- lime -->
</series>
<!-- ...(etc) -->
</elements>
</pieChart>