Hi,
When I open an article and click the edit icon next to Auther, I am redirected to K2 Users.
When I select a user I get the message:
The entry "Super User" was added to the list. (Or any other User I choose)
It looks so good
But,
There is a red button in the upper right named "Close" - nothing happens when I click on it.
It is subordinate to which user I choose as Author, the result is the same.

I can only get back to the article by using the browser's 'back' button.
When I return to the article, no Author has been added.

I use:
Joomla v. 3.9.11
K2 v. 2.9.1

What can I do to solve the problem?
Preferably an easy way since I'm not a programmer.

Thanks in advance :-)