Temporary workaround to fix 'checkbox save'. Problem: no 'unchecked' values!!! TG not tested.

extension/Classes/Core/Helper/HelperFormElement.php

@@ -213,11 +213,10 @@ class HelperFormElement {
     public static function prependFormElementNameCheckBoxMulti($field, $index, $appendArray = false) {
 //        return '_' . $index . '_' . $field;
 
-        $key = '_' . $field;
         if ($appendArray) {
-            $key .= '[]';
+            return $field . '[]';
         }
-        return $key;
+        return $field;
     }
 
     /**

extension/Classes/Core/Store/FillStoreForm.php

@@ -257,7 +257,7 @@ class FillStoreForm {
             switch ($formElement[FE_TYPE]) {
                 case FE_TYPE_CHECKBOX:
                     // Checkbox Multi: collect values
-                    $val = $this->collectMultiValues($clientFieldName, $clientValues);
+                    $val = $this->collectCheckBoxValues($clientFieldName, $clientValues);
                     if ($val !== false) {
                         $clientValues[$clientFieldName] = $val;
                     }
@@ -352,21 +352,23 @@ class FillStoreForm {
      *
      * @return string
      */
-    private function collectMultiValues($clientFieldName, array $clientValues) {
+    private function collectCheckBoxValues($clientFieldName, array $clientValues) {
 
-        $checkboxKey = HelperFormElement::prependFormElementNameCheckBoxMulti($clientFieldName, '', false);
+        // Check for Single
+//        $checkboxKey = HelperFormElement::prependFormElementNameCheckBoxMulti($clientFieldName, '', false);
 
-        if (!isset($clientValues[$checkboxKey])) {
-            return '';
-        }
+        if (isset($clientValues[$clientFieldName])) {
 
-        // single: no array, multi: array
-        if (!is_array($clientValues[$checkboxKey])) {
-            return false;
+            if (is_array($clientValues[$clientFieldName])) {
+                return implode(',', $clientValues[$clientFieldName]);
+            }
+
+            return $clientValues[$clientFieldName];
         }
 
+        return false;
+
 
-        return implode(',', $clientValues[$checkboxKey]);
 
         // For templateGroups: all expanded FormElements will be tried to collect - this fails for not submitted fields.
         // Therefore skip not existing clientvalues.