Chart won´t load using json

I try to load a chart with json using

    mychart.load(url, "json");

but got an XML parsing error. My web server log shows:

So everything seams OK but won´t work. If I try load an XML everything is OK.

What´s wrong with my json or with my code???

Unfortunately the problem cannot be reconstructed locally.
Could you please, provide a simplified complete demo, where the problem can be reconstructed.
Here you can find a tutorial about creating a complete demo: … pport.html