HTTP Error 414. The request URL is too long

For me, changing type:"get" to type:"post" worked, as get reveals all queries and hence make it bigger url.
Just change type from get to post.
This should help. :)

According to this question the maximum practical length of a URL is 2000 characters. This isn't going to be able to hold a massive Wikipedia article like you're trying to send.

Instead of putting the data on the URL you should be putting it in the body of a POST request. You need to add a data value to the object you're passing to the ajax function call. Like this:

function editAbout(){

    var about=escape( $("#editorAbout").text());
    $.ajax({
        url: "Allcammand.aspx?cmd=EditAboutCompany&idCompany="+getParam("idCompany"),
        type:"post",
        async: false,
        data: {
            about: about
        },
        success: function(response){                                       
        },
        error:function(xhr, ajaxOptions, thrownError){alert(xhr.responseText); ShowMessage("??? ?? ?????? ??????? ????","fail");}
    });
}