See "Error: Permission denied to access property 'handler'".
You can no longer invoke the target-page's jQuery like that.
(Note that in @grant none mode (the default as of GM 2), unsafeWindow is the same as window... But, then you can't use GM_ functions.)
@require your own copy of jQuery; it will not conflict with the page's and will load faster, to boot.
Do not use unsafeWindow for things like this (or at all, if you can help it), and $(document).ready() is also almost never needed for Greasemonkey scripts.
Your (new) sample script would merely be:
// ==UserScript==
// @name Dimi Test
// @namespace Dimi
// @version 1
// @grant GM_xmlhttpRequest
// @include about:addons
// @include http://*.myDomain.*/*
// @require http://ajax.googleapis.com/ajax/libs/jquery/2.1.0/jquery.min.js
// ==/UserScript==
$("body").prepend ('<h1>Hello World!</h1>');
And you can then mix GM_ functions and your instance of jQuery with no problems.
Note: The question script has // @include about:addons.
Greasemonkey scripts will not work on the about:addons page, by design.