I know this thread is old. But I was running into this issue and was finally able to resolve it. I thought I'd share my solution in case anyone else is still having this problem.

For me, I was missing a RouteConfig.cs in my project.

Here is the code I used (sorry for the formatting, I am unable to load the full reply page, so I'm stuck with the quick reply box and don't know the formatting syntax):
using DotNetNuke.Web.Mvc.Routing;

namespace ModuleNamespace
{
public class RouteConfig : IMvcRouteMapper
{
public void RegisterRoutes(IMapRoute mapRouteManager)
{
mapRouteManager.MapRoute("ModuleName", "ModuleName", "{controller}/{action}", new[]
{"ModuleNamespace.Controllers"});
}
}
}


Once I added that file in, I no longer got the extra HTML in my responses. Without the mapping, I was getting my JSON and the page load source, which was causing some database connection issues for me.

Hope this helps!

So i used this line in the ajax response but i feel like its super hacky, does can anyone help!?

...
success: function (responseWithDNNdingleberries) {
var jsonResponse = responseWithDNNdingleberries.substring(0, responseWithDNNdingleberries.indexOf(""));
var data = JSON.parse(jsonResponse);
...
}