After starting the module administration I have a text view (no html) , I can not save it and display an error: Fatal error: Call to undefined method K2HelperHTML::loadjQuery() in /web/htdocs2/aviation24pl/home/www/administrator/components/com_k2/elements/items.php on line 29.
How to remove it?
I update joomla to new version.
This module k2 Extra Slider is from 2013 Year (ver. 3.1.0) !!! Bought in 2016 from you!

Same exact error with different module (K2 News Module) and template (SJ ExpNews) smartaddons devs need to update modules for k2 2.8.0 and template overrides in template folder.
I have solved this by adding several lines of code to my template com_k2 folder and it fixed it.
www.smartaddons.com/forum/556-sj-expnews/33960-sj-mega-news-for-k2-fails-to-open-module-for-edit

Cyrus .. Please write what you added to the code line and in which file ?

Boro... my issue is with different template and different module, you cant apply same fix it is related to different files. It is gonna break your site. But the issue is similar. Ask devs to update template and module that is the right way.

Thanks but Of course, the only problem is to contact them

Well all or the problems I have fixed myself, didn't even bothered to log in here with account I have purchased membership. I have got what I needed good looking template and after that I have replaced 80% of the modules and scripts that came with template.
And now this error with K2 if you have read changelog k2 has made some under the hub changes that breaks everything.
Right now my mission is to completely remove K2 component from my site (to much hassle over it) but need to find module similar to it K2 News Module because it is nice.
Try sending message to the anhqn moderator he seems nice.