ASP.NET MVC controller return JSON on error. Not consistent on dev machine vs. dev server

Question

I have a MVC controller returning Task in a FileUpload function. (I use a Controller in stead of ApiController to make the file upload work properly)

My problem is whenever an exception is thown (for instance a file not being on a white list of allowed file types), my code behaves differently when debugging compared to code on a dev server I have.

My code:

//Exception is thown, message is put into the errorMessage parameter
Response.StatusCode = 500;
Response.StatusDescription = errorMessage;
Response.ContentType = "application/json";
return Json(new { Type = "Error", Message = errorMessage });

On the dev server, I get HTML result

I can't find any significant differences in web.configs Does anyone know where to investigate ? This might be a simple configuration thing, but I can't figure this out.