Add Task General Error 6500

Add Task General Error 6500

Hello, 

I'm trying to create a task following the guidelines on the API spec, but I can't get it to work. I tried several methidsbut I'm not getting significant enough infomation to figure out what's going on.

I tried POSTing the required fields (with and without &flag=internal attached) to: https://projectsapi.zoho.com/restapi/portal/xxxxxxxx/projects/xxxxxxxxxxxxxxx/tasks/?authtoken=xxxxxxxxxxxxxxxxxxx

Payload:
{
    "person_responsible": xxxx,
    "tasklist_id": xxxxxxxxxxxxxxxxxxxx,
    "name": "Test task",
    "start_date": "11-13-2018",
    "end_date": "11-14-2018",
    "duration": 1,
    "duration_type": "days",
    "priority": "Medium",
    "description": "test description",
    "uploaddoc": "",
    "start_time": "13:00",
    "end_time": "13:00",
    "owner_work": [],
    "work_type": "work_in_percentage",
    "custom_fields": {},
    "completed_on": "",
    "reminder": "",
    "json_string": {}
}

Which yields...
{
    "error": {
        "code": 6831,
        "message": "Input Parameter Missing"
    }
}

As far as I can tell, all required parameters are there. 
{
    "error": {
        "code": 6832,
        "message": "Input Parameter Does not Match the Pattern Specified"
    }
}

Via POST:
{
    "error": {
        "code": 6500,
        "message": "General Error"
    }
}


Are all fields required?
Which fields are optional and what do they default to?
Which is the preferred method: query string or payload?
Which HTTP method is the correct one?
Do you have a working sample URL/payload which I can work off of?

As far as I can tell the API spec doesn't answer this. Any help would be appreciated, thank you.