C# - Can't Load JSON Into A Jstree From WCF Service (or How To Find What's Wrong When Jstree Stays On "loading...")

I am trying to get jStree (jQuery tree control) working in my ASP.NET (C#) page. I'm using a WCF service to return a test string of JSON. Opening the WCF service URL in the browser works fine - If I go to the URL (http://localhost/website/GetTree) I get the JSON (looks just like opening XML in browser, with syntax highlighting and collapsible tab thingys)

<string xmlns="http://schemas.microsoft.com/2003/10/Serialization/">{"attr": { "id": 2, "rel": "default" }, "data": "A node", "children": [ { "data": "Child 1", "attr": { "id": 43, "rel": "document" } }, {"data": "Child 2", "attr": { "id": 25, "rel": "document"} } ] }</string>

jstree itself works fine - if I put the same JSON in "manually" in my document.ready like so:

"json_data": {
"data":
[{
"attr": { "id": 2, "rel": "default" },
"data": "A node",
"children":
[{
"data": "Child 1", "attr": { "id": 43, "rel": "document" }
},{
"data": "Child 2", "attr": { "id": 25, "rel": "document"}
}]},{
"attr": { "id": "li.node.id", "rel": "document" },
"data": {
"title": "Long format demo",
"attr": { "href": "#" }
}}]},


I get the tree appearing with those nodes in it.

But, combining the two:

"json_data": {
"ajax": {
"url": "GetTree",
"data": function (n) {
return {
};
}
}
},

fails. The jstree shows up as just the word "loading..." and an animated loading gif. And it does hit a breakpoint inside the WCF service method (just like browsing to it does).

A: An idea where I've gone wrong?

B: An idea how I find out what's wrong? jstree documentation was no , and I can't see any error messages.

Update: solved

[Code]....