Received "An illegal choice has been detected. Please contact the site administrator" while creating node with taxonomy field via service3 drupal7
I was having the same issue. After reading this I was able to figure out the problem by doing a dsm($_POST) on hook_node_update() and inspecting the array and figuring out what it expects.

In my case my array looked like this:

'field_term_subtype[und]' =&gt; '166',


In your case it would be:

{
    "title": "your title",
    "type": "page",
    "field_category": {
        "und": 6246
    }
}