Fixes issue #3740 for horizontal layouts

extensions/bootstrap/ActiveField.php

@@ -356,7 +356,7 @@ class ActiveField extends \yii\widgets\ActiveField
             $config['horizontalCssClasses'] = $cssClasses;
             $config['wrapperOptions'] = ['class' => $cssClasses['wrapper']];
             $config['labelOptions'] = ['class' => 'control-label ' . $cssClasses['label']];
-            $config['errorOptions'] = ['class' => 'help-block ' . $cssClasses['error']];
+            $config['errorOptions'] = ['class' => 'help-block help-block-error ' . $cssClasses['error']];
             $config['hintOptions'] = ['class' => 'help-block ' . $cssClasses['hint']];
         } elseif ($layout === 'inline') {
             $config['labelOptions'] = ['class' => 'sr-only'];