ASP.NET Core WebAPI 500 Internal Error in IIS 7.5

I am trying to get this WebAPI to work. Well, use IIS. Everything works fine in IIS express, but when I publish it, especially 1 api request doesn't work. The url I am trying to access API/[Controller]/{date}/{integer}. I keep getting a 500 server error. Another API/[Controller]/{date}working line of mine .

Here is my API controller:

[Route("api/[controller]")]
    public class PostingsController : Controller
    {
        // GET: api/Postings/5
        [HttpGet("{date}")]
        public string Get(string date)
        {
            return date;
        }

        // GET api/Postings/20160407/2
        [HttpGet("{date}/{modeID}")]
        public List<TruckPosting> Get(string date, int modeID)
        {
            TruckPosting tp = new TruckPosting();
            List<TruckPosting> truckPostings = tp.GetTruckPostings(date, modeID);
            return truckPostings;
        }
    }

Could the reason be that I'm returning a List<>? I'm baffled considering it works fine in IIS Express in VS.

edit

Here is my startup.cs page:

public void ConfigureServices(IServiceCollection services)
        {
            services.AddMvc();
        }

public void Configure1(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
            {
                app.UseIISPlatformHandler();
                app.UseDefaultFiles();
                app.UseStaticFiles();
                app.UseFileServer(true);
                app.UseMvc();
            }

            // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
            {
                app.Map("/appRoot", (app1) => this.Configure1(app1, env, loggerFactory));
            }