Upgraded site won't run Tasks that index pages

We have a site we’ve upgraded to 9.1.2 from 8.5.9 that won’t run Tasks that include page indexing like Generate Sitemap or Reindex Content.

The dashboard page returns the error:

Failed to send an update.

If run in the command line we get:

Generation of sitemap.xml started.

In HandleMessageMiddleware.php line 128:
  Handling "Concrete\Core\Command\Task\Command\ExecuteConsoleTaskCommand" failed: Failed to send an update.  
In Hub.php line 104:
  Failed to send an update.  

In HttpClientTrait.php line 451:
  Invalid URL: scheme is missing in "". Did you forget to add "http(s)://"?

Any ideas where to look for a solution?

Interesting @katalysis . Can you send along the site’s configuration? Some additional information, will help. I’ll make sure an engineer takes a look at this thread.

I’m going to pm the config file to you @jessicadunbar.

I’ve taken this forward a bit today and can now stop the problem if I remove the following from my concrete.php generated overrides file:

 'messenger' => [
        'consume' => [
            'method' => 'app',
    'processes' => [
        'scheduler' => [
            'enable' => false,
        'logging' => [
            'method' => 'none',
            'file' => [
                'directory' => '',
    'notification' => [
        'server_sent_events' => true,
        'mercure' => [
            'default' => [
                'connection_method' => 'single_secret_key',

This doesn’t feel like a complete solution though.

It sounds like you might have enabled server side events without providing a mercure url. If you’re trying to enable mercure based server side events then make sure to provide a url to the mercure instance. If not then you can remove the concrete.notification.server_sent_events config item (or set it to false instead of true) to disable this functionality either manually or through the system/notification/events dashboard page.