update

Civi/FormProcessor/Type/OptionGroupType.php

@@ -100,6 +100,15 @@ class OptionGroupType extends AbstractType implements OptionListInterface {
   public function normalizeValue($value) {
     $this->loadOptions();
     $multiple = $this->configuration->get('multiple') ? true : false;
+
+    // Correct array values when field is not multiple.
+    // this could be caused for example by a drupal webform with
+    // a checkbox field. The value is submitted as an array, altough it contains
+    // only one value.
+    if (!$multiple && is_array($value) && count($value) == 1) {
+      $value = reset($value);
+    }
+
     if ($multiple && is_array($value)) {
       $return = array();
       foreach($value as $item) {