I ran into this issue today trying to load some simple data from a php script. I was positive I was only sending name/value pairs and nothing else.  I had set the loader.dataFormat property to URLLoaderDataFormat.VARIABLES, and using Charles I can see that my data was arriving fine.
The solution in my case was simple: Â I was adding a leading ‘&’ in the variables string, which URLLoader doesn’t appreciate. So instead of this, which was a habit picked up with AS 2:
echo "&result=success"
You just need to do:
echo "result=success"
You still need the ampersand to separate multiple name/value pairs though.
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yah… havin’ this error a month ago
If you don’t know what it can drive you crazy. (happen to me:)