How can I determine the cause of the Internal Server Error resulting from my Ajax call?

Question

In my ASP.NET MVC app, with this AJAX call:

$.ajax({
    type: 'GET',
    url: '@Url.Action("GetUnitReportPairVals", "Home")',
    data: { unit: unitval, report: rptval }, // data: model,
    contentType: 'application/json',
    cache: false,
    success: function (result) {
        alert(result);
    },
    error: function (result) {
        alert('failed');
        alert(result);
    }
});

...calling this Controller method:

public JsonResult GetUnitReportPairVals(string unit, string report)
{
    int rptId = GetReportIDForName(report);

    DataTable UnitReportPairEmailValsDT = new DataTable();
    string qry = string.Format(SQL.UnitReportPairEmailQuery, unit, rptId);
    UnitReportPairEmailValsDT = SQL.ExecuteSQLReturnDataTable(
        qry, 
        CommandType.Text,
        null
        );

    var model = UnitReportPairEmailValsDT;
    return Json(model);
}

I see the "failed" alert, and the "result" object has no end of data; here's just the tip of the iceberg:

It seems the key thing is a Status of 500, and Status text of "Internal Server Error"

So I commented out those two lines and added in "debugger" in their stead (as suggested by nurdyguy here):

$.ajax({
    . . .
    error: function (result) {
        debugger;
        //alert('failed');
        //alert(result);
    }
});

...yet I don't see what that gets me. Putting the ajax call through its paces just shows nothing in the browser; using Chrome DevTools, I can step into it, but once I get to the "debugger" line, it doesn't do me any favors. Even F11 from that line does nothing.

So how can I get to the bottom of what's causing this 500/Internal Server Error?

UPDATE

Since I have commented out the "JSON" lines from the jQuery AJAX call var model = JSON.stringify({}) and contentType: 'application/json', should I also change the Controller return type from JsonResult to ActionResult and return View(model)?

UPDATE 2

I am getting a success from my AJAX method now with this, incorporating several suggestions from y'all and changing the model from a DataTable member to a generic list of string:

// Model (C#)
public class UnitReportPairModel
{
    public List<String> UnitReportPairEmailVals { get; set; }
    . . .
}   

// Controller (C#)
public JsonResult GetUnitReportPairEmailAddresses(string unit, string report)
{
    UnitReportPairModel model = new UnitReportPairModel();
    try
    {
        int rptId = GetReportIDForName(report);

        DataTable UnitReportPairEmailValsDT = new DataTable();
        string qry = string.Format(SQL.UnitReportPairEmailQuery, unit, rptId);
        UnitReportPairEmailValsDT = SQL.ExecuteSQLReturnDataTable(
            qry,
            CommandType.Text,
            null
            );

        List<String> emailAddresses = UnitReportPairEmailValsDT
                     .AsEnumerable()
                     .Select(row => row.Field<string>("EmailAddr"))
                     .ToList();

        model.UnitReportPairEmailVals = emailAddresses;
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex.Message);
    }
    return Json(model, JsonRequestBehavior.AllowGet);
}

// View (jquery)
var model = JSON.stringify({ unit: unitval, report: rptval });
    $.ajax({
        type: 'GET',
            url: '@Url.Action("GetUnitReportPairEmailAddresses", 
    "UnitReportPair")',
            data: { unit: unitval, report: rptval }, // data: model,
            contentType: 'application/json',
            cache: false,
            success: function (result) {
                alert('success');
                alert(result.data);
            },
            error: function () {
                alert('failure');
            }
        });

UPDATE 2

Once I pieced together several different answers to this and related questions, I wrote up a tip on how to do this here.

Answer 1

Since it is a GET action method, if you are returning json data, you should explicitly specify JsonRequestBehaviour.AllowGet.

This should fix it.

return Json(model,JsonRequestBehavior.AllowGet);

If your action method is decorated with [HttpPost] and you are making a POST call, you do not need to explicitly specify it, return Json(model) will work fine.

Here is a more detailed explanation about the same.