But during debug, the method is not even called.

It won't because your post signature does not match the method in your controller

If you change your controller method to

public async Task<ActionResult> GetContratos(Object obj)

does your post then hit the controller method?

Nope, i change to this:

[HttpPost]
    public ActionResult GetContratos(object obj)
    {
        //some code
        return Json(new
        {
            draw = 1,
            recordsTotal = 1,

        }, JsonRequestBehavior.AllowGet);
    }

After disable the redirect to 404 page, i got this response:

Server Error in '/' Application. Sequence contains no elements Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidOperationException: Sequence contains no elements

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[InvalidOperationException: Sequence contains no elements] System.Linq.Enumerable.First(IEnumerable`1 source) +455 Umbraco.Core.Configuration.UmbracoSettings.ContentErrorsElement.get_Error404Collection() in D:\a\1\s\src\Umbraco.Core\Configuration\UmbracoSettings\ContentErrorsElement.cs:13 Umbraco.Web.Routing.ContentFinderByConfigured404.TryFindContent(PublishedRequest frequest) in D:\a\1\s\src\Umbraco.Web\Routing\ContentFinderByConfigured404.cs:57 Umbraco.Web.Routing.PublishedRouter.HandlePublishedContent(PublishedRequest request) in D:\a\1\s\src\Umbraco.Web\Routing\PublishedRouter.cs:459 Umbraco.Web.Routing.PublishedRouter.FindPublishedContentAndTemplate(PublishedRequest request) in D:\a\1\s\src\Umbraco.Web\Routing\PublishedRouter.cs:401 Umbraco.Web.Routing.PublishedRouter.PrepareRequest(PublishedRequest request) in D:\a\1\s\src\Umbraco.Web\Routing\PublishedRouter.cs:135 Umbraco.Web.UmbracoInjectedModule.ProcessRequest(HttpContextBase httpContext) in D:\a\1\s\src\Umbraco.Web\UmbracoInjectedModule.cs:138 System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +223 System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step) +232 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +100

does it reach the method before erroring? if not then you have something configured incorrectly somewhere as it still appears to be trying to use your 404 settings

I did a clean install, same problem.

https://postimg.cc/w305QT7v

what does your view and js look like where you are doing the post

Im testing using postman

{ "draw": 1, "start": 0, "length": 10, "filterValue": "test" }

I don't think postman will work posting to a surface controller, although never tried myself. I will give it a try.