🐛 [#4022] Handle serialized args to celery task

Backport-of: #4023

src/openforms/registrations/tasks.py

@@ -102,7 +102,7 @@ def pre_registration(submission_id: int, event: PostSubmissionEvents) -> None:
     ignore_result=False,
     once={"graceful": True},  # do not spam error monitoring
 )
-def register_submission(submission_id: int, event: PostSubmissionEvents) -> None:
+def register_submission(submission_id: int, event: PostSubmissionEvents | str) -> None:
     """
     Attempt to register the submission with the configured backend.
 
@@ -279,7 +279,10 @@ def register_submission(submission_id: int, event: PostSubmissionEvents) -> None
         submission,
     )
 
-    if config.wait_for_payment_to_register and event.on_payment_complete:
+    if (
+        config.wait_for_payment_to_register
+        and event == PostSubmissionEvents.on_payment_complete
+    ):
         submission.payments.update(status=PaymentStatus.registered)
 
     submission.save_registration_status(RegistrationStatuses.success, result or {})

src/openforms/registrations/tests/test_registration_hook.py

@@ -1,6 +1,7 @@
 """
 Test the registration hook on submissions.
 """
+
 from datetime import timedelta
 from unittest.mock import patch
 
@@ -17,6 +18,7 @@
 from openforms.forms.models import FormRegistrationBackend
 from openforms.logging.models import TimelineLogProxy
 from openforms.submissions.constants import PostSubmissionEvents, RegistrationStatuses
+from openforms.submissions.public_references import get_random_reference
 from openforms.submissions.tests.factories import SubmissionFactory
 
 from ..base import BasePlugin
@@ -345,6 +347,27 @@ def test_registration_backend_invalid_options(self):
         ):
             register_submission(submission.id, PostSubmissionEvents.on_retry)
 
+    def test_calling_registration_task_with_serialized_args(self):
+        submission = SubmissionFactory.create(
+            completed=True,
+            public_registration_reference=get_random_reference(),
+            with_completed_payment=True,
+            form__registration_backend="email",
+            form__registration_backend_options={"to_emails": ["[email protected]"]},
+        )
+
+        with patch(
+            "openforms.registrations.tasks.GlobalConfiguration.get_solo",
+            return_value=GlobalConfiguration(wait_for_payment_to_register=True),
+        ):
+            register_submission(
+                submission.id, str(PostSubmissionEvents.on_payment_complete)
+            )
+
+        submission.refresh_from_db()
+
+        self.assertEqual(submission.registration_status, RegistrationStatuses.success)
+
 
 class NumRegistrationsTest(TestCase):
     @patch("openforms.plugins.registry.GlobalConfiguration.get_solo")