diff --git a/webapp/src/DataTransferObject/Shadowing/LanguageEvent.php b/webapp/src/DataTransferObject/Shadowing/LanguageEvent.php
index aa5e799f4b..999bca1eed 100644
--- a/webapp/src/DataTransferObject/Shadowing/LanguageEvent.php
+++ b/webapp/src/DataTransferObject/Shadowing/LanguageEvent.php
@@ -4,7 +4,11 @@
 
 class LanguageEvent implements EventData
 {
+    /**
+     * @param list<string>|null $extensions
+     */
     public function __construct(
         public readonly string $id,
+        public readonly ?array $extensions = null,
     ) {}
 }
diff --git a/webapp/src/Service/ExternalContestSourceService.php b/webapp/src/Service/ExternalContestSourceService.php
index bc53ca6aec..73372eb6fd 100644
--- a/webapp/src/Service/ExternalContestSourceService.php
+++ b/webapp/src/Service/ExternalContestSourceService.php
@@ -381,11 +381,10 @@ protected function importFromCcsApi(array $eventsToSkip, ?callable $progressRepo
             };
 
             while (true) {
-                // A timeout of 0.0 means we get chunks immediately and the user
-                // can cancel at any time.
                 try {
                     $receivedData = false;
-                    foreach ($this->httpClient->stream($response, 0.0) as $chunk) {
+                    // Get a timeout chunk after 1 second so we don't hang indefinitely.
+                    foreach ($this->httpClient->stream($response, 1.0) as $chunk) {
                         // We first need to check for timeouts, as we can not call
                         // ->isLast() or ->getContent() on them.
                         if (!$chunk->isTimeout()) {
@@ -833,6 +832,10 @@ protected function validateLanguage(Event $event, EventData $data): void
             ]);
         } else {
             $this->removeWarning($event->type, $data->id, ExternalSourceWarning::TYPE_DATA_MISMATCH);
+
+            $toCheck = ['extensions' => $data->extensions];
+
+            $this->compareOrCreateValues($event, $data->id, $language, $toCheck);
         }
     }
 
@@ -1432,6 +1435,10 @@ protected function importSubmission(Event $event, EventData $data): void
                 $zipUrl = $data->files[0]->href;
                 if (preg_match('/^https?:\/\//', $zipUrl) === 0) {
                     // Relative URL, prepend the base URL.
+                    // If the base URL ends with a slash and the zip URL starts with one, remove the slash.
+                    if (str_ends_with($this->basePath, '/') && str_starts_with($zipUrl, '/')) {
+                        $zipUrl = substr($zipUrl, 1);
+                    }
                     $zipUrl = ($this->basePath ?? '') . $zipUrl;
                 }
 
@@ -1461,26 +1468,31 @@ protected function importSubmission(Event $event, EventData $data): void
                     }
 
                     if ($submissionDownloadSucceeded) {
-                        try {
-                            $response = $this->httpClient->request('GET', $zipUrl);
-                            $ziphandler = fopen($zipFile, 'w');
-                            if ($response->getStatusCode() !== 200) {
-                                // TODO: Retry a couple of times.
+                        $tries = 1;
+                        do {
+                            try {
+                                $response = $this->httpClient->request('GET', $zipUrl);
+                                $ziphandler = fopen($zipFile, 'w');
+                                if ($response->getStatusCode() !== 200) {
+                                    $this->addOrUpdateWarning($event, $data->id, ExternalSourceWarning::TYPE_SUBMISSION_ERROR, [
+                                        'message' => "Cannot download ZIP from $zipUrl after trying $tries times",
+                                    ]);
+                                    $submissionDownloadSucceeded = false;
+                                    // Sleep a bit before retrying
+                                    sleep(3);
+                                }
+                                $tries++;
+                            } catch (TransportExceptionInterface $e) {
                                 $this->addOrUpdateWarning($event, $data->id, ExternalSourceWarning::TYPE_SUBMISSION_ERROR, [
-                                    'message' => 'Cannot download ZIP from ' . $zipUrl,
+                                    'message' => "Cannot download ZIP from $zipUrl after trying $tries times: " . $e->getMessage(),
                                 ]);
+                                if (isset($ziphandler)) {
+                                    fclose($ziphandler);
+                                }
+                                unlink($zipFile);
                                 $submissionDownloadSucceeded = false;
                             }
-                        } catch (TransportExceptionInterface $e) {
-                            $this->addOrUpdateWarning($event, $data->id, ExternalSourceWarning::TYPE_SUBMISSION_ERROR, [
-                                'message' => 'Cannot download ZIP from ' . $zipUrl . ': ' . $e->getMessage(),
-                            ]);
-                            if (isset($ziphandler)) {
-                                fclose($ziphandler);
-                            }
-                            unlink($zipFile);
-                            $submissionDownloadSucceeded = false;
-                        }
+                        } while ($tries <= 3 && !$submissionDownloadSucceeded);
                     }
 
                     if (isset($response, $ziphandler) && $submissionDownloadSucceeded) {