Hello people,
I had installed a year ago the newsworth template.
At the front page there is a k2 module content with subtemplate video player (see the picture).
You click on the thumbs on the right side and the video of the specific articles appears. I don;t know the reason but this functionality stopped and now always appeared the video of the first article.
I found the deafult.php of the video player template and started the debuging.
I found that all this happens because when the javascript add the format=JSON at the end of every article in order to get the JSON data of every article I get an error
(0) Call to undefined method K2ModelItemlist::prepareJSONItem()
Is there a way to solve this problem?

Thanks a lot!

k2 2.10.3
Joomla 3.9.19

Finally, the problem was a custom template that I created to html/com_k2.
When I assign the default template to the category the format=JSON is working perfect and the video player of course,
but when I assign my custom template the things not working.

So I have to see what might be wrong with my custom template. Do I have to add my custom template com_k2 to a file like xml in order to the articles assigned to this template to have json format?

Thanks for your help Fotis.

Well I think that there is a bug.
If you have an article asign to a category, that in display settings has disabled the item navigation then you cannot get the article in a json format and you get the error,
(0) Call to undefined method K2ModelItemlist::prepareJSONItem()

How can I send you a private message for more information?

Hello Fotis, I see that the json format at the k2 blog is working perfect.

But I install a new joomla 3.9.19 in php 7.4, with k2 component (K2 v2.10.4[Dev Build 20200604]) and newsworth template and nothing else (not 3rd party plugin) and it has the same behaviour as I described in my earlier post. I think the bug is at the newsworth template.

You can see the demo site here: www.tennisgreece.gr
credentials admin / admin

To my official website I had found a solution via css and display: none the navigation control. So, if you don't have time, don't look it at all.

Thanks