XXE successfully completed message was no longer shown, fixed it by using form POST together with customjs functions.

Introduced callback functionality which you can specify after the posting in order to be able to load the comments list again.

webgoat-lessons/xxe/src/main/resources/js/xxe.js

@@ -1,23 +1,17 @@
+webgoat.customjs.simpleXXE = function () {
+    var commentInput = $("#commentInputSimple").val();
+    var xml = '<?xml version="1.0"?>' +
+        '<comment>' +
+        '  <text>' + commentInput + '</text>' +
+        '</comment>';
+    return xml;
+}
+
+webgoat.customjs.simpleXXECallback = function() {
+    getComments('#commentsListSimple');
+}
+
 $(document).ready(function () {
-    $("#postCommentSimple").unbind();
-    $("#postCommentSimple").on("click", function () {
-        var commentInput = $("#commentInputSimple").val();
-        var xml = '<?xml version="1.0"?>' +
-            '<comment>' +
-            '  <text>' + commentInput + '</text>' +
-            '</comment>';
-        $.ajax({
-            type: 'POST',
-            url: 'xxe/simple',
-            data: xml,
-            contentType: "application/xml",
-            dataType: 'xml',
-            complete: function (data) {
-                $("#commentInputSimple").val('');
-                getComments('#commentsListSimple')
-            }
-        })
-    });
     getComments('#commentsListSimple');
 });