H5P related cookie

This is no cookie, but a localStorage property that is used to track the same user across different tasks using the xAPI statements. The user may choose to disallow/disable this, in which case the site owner will not be able to tell that the same user answered two different tasks.
The code can be seen here: https://github.com/h5p/h5p-php-library/blob/0922db6fab0ee17c35781d557473e7d1fd5c0038/js/h5p-x-api-event.js#L204

OK, thanks! Last Question: How can I disable it? I searched for that option, but I can‘t find anything.

There is no way of preventing it from being set as long as localStorage is enabled in the browser. However, the property isn’t really used unless you store the xAPI statements.