Data structure returned is not convertible in the requested format. Try to call this method with the parameters '&format=original&serialize=1'; you will get the original php data structure serialized. The data structure looks like this:
$data = array (
'Limitation' =>
array (
0 => 'Multi dimensional arrays is only supported by format=JSON',
1 => 'Known limitation',
),
'Second Dimension' =>
array (
0 => true,
1 => false,
2 => 1,
3 => 0,
4 => 152,
5 => 'test',
6 =>
array (
42 => 'end',
),
),
);