I am still struggling with jquery on my site. When comparing one site with another, I noticed that on the 'problem' site /media/k2/assets/js/k2.frontend.js?v=2.8.0&sitepath=/ is loaded before jQuery scripts are loaded. Thus the K2-javascript can't execute jQuery, because it hasn't found it yet. How can I change this so that k2.frontend.js is the last script loaded, after jQuery?

In addition: I've tried various settings. If I turn off jQuery in the K2 settings, editing an article beaches a mess. The different tabs for images, galleries etc. disappear. Only if I turn on the land Jquery option in K2 it's fine. But then the SIG Pro window overwrites the edit window, and doens't open as a modal window. Thus you loose your text and you can't close the SIG window.

I would suggest either loading K2 on a template basis (use Joomla"s API) or you can use jQuery Easy.

I did have jQuery Easy already installed, but that doesn't help. I had to turn it off, to make the tabs appear in K2.
Strangely enough I created a testsite from scratch with the same Yootheme template, and there it's fine.
What do you mean with the template and Joomla api?

That would apply if you are building your template from scratch.
Since this is a commercial template, you need to address this issue to YooTheme as well and provide them with both links. This looks more like a template issue rather than an issue with extensions.