UPD: made interface for office useful.

dav_events/views/events.py

@@ -413,7 +413,7 @@ class EventRegistrationsView(EventPermissionMixin, generic.DetailView):
                 return self.render_to_response(self.get_context_data(create_participant_form=form))
         else:
             messages.error(request, 'unsupported action: {}'.format(action))
-        return HttpResponseRedirect(reverse('dav_events:registrations', kwargs={'pk': event.pk}))
+        return self.render_to_response(self.get_context_data())
 
     @method_decorator(login_required)
     def dispatch(self, request, *args, **kwargs):