diff --git a/cvat/apps/engine/views.py b/cvat/apps/engine/views.py
index d507d7e9..64c7dfe3 100644
--- a/cvat/apps/engine/views.py
+++ b/cvat/apps/engine/views.py
@@ -1221,10 +1221,14 @@ class TaskViewSet(viewsets.GenericViewSet, mixins.ListModelMixin,
         elif job.is_queued:
             response = { "state": "Queued" }
         elif job.is_failed:
-            response = { "state": "Failed", "message": job.exc_info }
+            # FIXME: It seems that in some cases exc_info can be None.
+            # It's not really clear how it is possible, but it can
+            # lead to an error in serializing the response
+            # https://github.com/opencv/cvat/issues/5215
+            response = { "state": "Failed", "message": job.exc_info or "Unknown error" }
         else:
             response = { "state": "Started" }
-            if 'status' in job.meta:
+            if job.meta.get('status'):
                 response['message'] = job.meta['status']
             response['progress'] = job.meta.get('task_progress', 0.)