Updated to $_REQUEST.

tested from my end.

data/web/inc/ajax/sieve_validation.php

@@ -4,14 +4,14 @@ header('Content-Type: application/json');
 if (!isset($_SESSION['mailcow_cc_role'])) {
   exit();
 }
-if (isset($_GET['script'])) {
+if (isset($_REQUEST['script'])) {
   $sieve = new Sieve\SieveParser();
   try {
-    if (empty($_GET['script'])) {
+    if (empty($_REQUEST['script'])) {
       echo json_encode(array('type' => 'danger', 'msg' => $lang['danger']['script_empty']));
       exit();
     }
-    $sieve->parse($_GET['script']);
+    $sieve->parse($_REQUEST['script']);
   }
   catch (Exception $e) {
     echo json_encode(array('type' => 'danger', 'msg' => $e->getMessage()));