-        lecture_data = self._download_json(
-            '%s/site/api/lecture/%s?format=json' % (base_url, lecture_id),
-            lecture_id)['lecture'][0]
+        try:
+            lecture_data = self._download_json(
+                '%s/site/api/lecture/%s?format=json' % (base_url, lecture_id),
+                lecture_id)['lecture'][0]
+        except ExtractorError as e:
+            if isinstance(e.cause, compat_HTTPError) and e.cause.code == 403:
+                msg = self._parse_json(
+                    e.cause.read().decode('utf-8'), lecture_id)
+                raise ExtractorError(msg['detail'], expected=True)
+            raise