recv() failed (104: Connection reset by peer) while reading response header from upstream

In most cases, this error is related to Nginx configuration.

Just Google it, there's a lot of answers on this.

However, I don't have any access to the list of these subscribers, nor any capacity to suppress subscribers from this distribution list.

Does this exist at all?

No

Is it foreseen?

Not unless somebody pays for it ;)

a WalledGarden concept that I don't see as existing (any more?) in my version. Is there a means to achieve this result in Elgg 2.3.7?

Restrict pages to logged-in users == Walled Garden

More on http://learn.elgg.org/en/stable/guides/walled-garden.html

I thank you very much for your prompt answer.

Does this mean that I need to develop my own plugin hook to make elements of information publicly visible? I would assume that the variable to be changed by the hook is the visibility of the information. Is this right? Does this variable have a name? What are the values it can have?

Does this mean that I need to develop my own plugin hook to make elements of information publicly visible?

That's right. You can start with this skeleton.

Does this variable have a name? What are the values it can have?

Just follow instructions and make something like this:

elgg_register_plugin_hook_handler('public_pages', 'walled_garden', 'my_plugin_walled_garden_public_pages');

function my_plugin_walled_garden_public_pages($hook, $type, $pages) {
   $pages[] = 'blog/group'; //For Blog plugin
   $pages[] = 'pages/group'; //For Pages plugin
   $pages[] = 'event_calendar/group'; //For Event calendar plugin
   
   return $pages;
}

Check out the full code with using the mentioned skeleton plugin.

But I think that it willn't work because Walled Garden removes ACCESS_PUBLIC and set ACCESS_LOGGED_IN default access level instead.

So 'blog/all' will be work but not 'blog/group' in this case.