diff -u b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php
--- b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php
+++ b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php
@@ -241,7 +241,15 @@
         $title = $form['source'][$key]['#title'];
         if (!isset($parsed_source['scheme'])) {
           if (!file_exists($source) || (!is_dir($source)) || (!is_readable($source))) {
-            $form_state->setErrorByName($key, $this->t('Unable to read from @title @source.', ['@title' => $title, '@source' => $source]));
+            $msg = [];
+            $msg[] = $this->t('Unable to read from @title @source.', [
+              '@title' => $title,
+              '@source' => $source]);
+            $error_message = [
+              '#theme' => 'item_list',
+              '#items' => $msg,
+            ];
+            $form_state->setErrorByName($key, $this->renderer->renderPlain($error_message));
           }
         }
         else {
@@ -252,17 +260,29 @@
           catch (TransferException $e) {
             if ($e->getResponse()) {
               $status = $e->getResponse()->getStatusCode();
-              $form_state->setErrorByName($key, $this->t('Unable to read from @title @source. Status code @status.', [
+              $msg = [];
+              $msg[] = $this->t('Unable to read from @title @source. Status code @status.', [
                 '@title' => $title,
                 '@source' => $source,
                 '@status' => $status,
-              ]));
+              ]);
+              $error_message = [
+                '#theme' => 'item_list',
+                '#items' => $msg,
+              ];
+              $form_state->setErrorByName($key, $this->renderer->renderPlain($error_message));
             }
             else {
-              $form_state->setErrorByName($key, $this->t('Unable to read from @title @source.', [
+              $msg = [];
+              $msg[] = $this->t('Unable to read from @title @source.', [
                 '@title' => $title,
                 '@source' => $source,
-              ]));
+              ]);
+              $error_message = [
+                '#theme' => 'item_list',
+                '#items' => $msg,
+              ];
+              $form_state->setErrorByName($key, $this->renderer->renderPlain($error_message));
             }
           }
         }