In first, elgg-plugin.php is not an Elgg 2 file but Elgg 3.

For custom Elgg 2 action you need to register it in your start.php.

Tank you but the problem is that the monthly cron is not triggered

I checked with my host. It is a shared plan, which does not support archive.

 It only supports the following DB engines. 

https://stackoverflow.com/questions/1458122/enabling-archive-engine

https://stackoverflow.com/questions/26996618/install-mariadb-archive-engine

Or use the suggested plugin, and delete the system log archives tables manually from your DB.

Don't forget about this.

Copy this file from:

vendor\elgg\elgg\views\default\elgg.js.php

To your custom plugin:

\mod\my_plugin\views\default\elgg.js.php

Comment this:

$elggDir->getPath("js/lib/ui.js"),

Add after:

foreach ($files as $file) {
   readfile($file);
   // putting a new line between the files to address https://github.com/elgg/elgg/issues/3081
   echo "\n";
}

Path to your custom ui.js (e.g. \mod\my_plugin\views\default\lib\ui.js):

readfile(dirname(__FILE__). '/lib/ui.js');
echo "\n";

Clean the caches.

Done.

Works great.

Thank you very much dear RVR

thank you! »digest« is exactly the term i was looking for :-) 
and hypenotifications would be the perfect plugin, but i did'nt found it yet, 
because i'm using an elgg 3 installation – is there still hope for me?

https://github.com/hypeJunction/Elgg3-hypeNotifications

great! now everything is fine and is running as expected. 
thanks again.