C# ASP.NET Ajax Communication Returns Large HTML

I'm a beginner at MVC.
The workspace is local.

$.ajax({
        type: "POST",
        url: "/aaa/bbb",
        data: {selectService:sendId},
        success:function(msg){
            alert(msg);

...

 [HttpPost]
public string bbb(string selectService)
{
    varmsg="aaa";
    return msg;
}

It's a method that usually works.
Maybe I've done something, but suddenly I get a HTML source that doesn't matter, not any string.

Until then, I had put a breakpoint on the controller side to check the contents, but it didn't get stuck either.
The ajax communication itself seems to be successful.(Alerts work)
The error side is unresponsive.

What can you think of?