diff -u b/core/modules/config_translation/src/Form/ConfigTranslationFormBase.php b/core/modules/config_translation/src/Form/ConfigTranslationFormBase.php
--- b/core/modules/config_translation/src/Form/ConfigTranslationFormBase.php
+++ b/core/modules/config_translation/src/Form/ConfigTranslationFormBase.php
@@ -218,7 +218,7 @@
         // Is the submitted value deemed not safe?
         if (!$is_safe) {
           // Retrieve field label and name attribute.
-          $field_name = 'translation[config_names][' . $name . '][' . $key;
+          $field_name = 'translation][config_names][' . $name . '][' . $key;
           // Field value is not safe, display error.
           $form_state->setErrorByName($field_name, $this->t('The submitted string contains disallowed HTML.'));
         }