Hmm...I have no clue how to do that.... I'll look for some documentation then...

Firebug Console shows this error:

The site with this error is a Quickinstall of the template with all extensions. I did install the template and K2 one by one at the old site and there it works, also in a non modal window.

So..why is "submitform" not defined in the one and defined in the other K2 installation? K2 is always the same.

Can you tell me where to define submitform (pressbutton) ??

Back the file beta.vivonet.de/media/k2/assets/js/k2.js and replace its contents with github.com/joomlaworks/k2/blob/master/media/k2/assets/js/k2.js

Let me know if that solved the issue.

No, unfortunately not.

I still need a solution. The development phase is almost over and the site will go online soon and some members have to write blogs from the frontend.

Backup.
Install the dev version which can be found here: getk2.org/assets/get/

No, that doesn't help. I don't believe it's a real K2 problem. It't probably a template problem.
First I installed the template and then K2 in our existing site. But the template configuration was to complicated to build up the site step by step. With this installation saving and closing in the frontend works.

So I set up a new Joomla installation with the templates quickinstall - a completely pre installed template with all extensions and predefined sites and here saving and closing K2 articles in the frontend doesn't work.
The only difference I see with firebug is the error message a few posts earlier. I think that's the problem but how to find out from where this error message comes and how to fix it?

If you think this is a template issue, you address this issue alongside with our conversation, to the template's developer.

It seems that the template developer is more a modeller as a developer. He is only able to handle support with getting admin access to the site. But this is not possible for security reasons.

To be honest that sounds fair.
Without seeing the issue there is little he can do. He can only guess.