As you can see in
github.com/getk2/k2/search?q=characterLimit2 there is no such function in K2.
Your site most likely had a modified K2 version and any changes done in K2's core files were lost when you upgraded to K2 latest.
Search for "characterLimit2" inside your template's html/com_k2/ files and if you don't find anything, then look in /modules and then /plugins. When you find it, replace "characterLimit2" with "characterLimit" and your site's pages will be restored.
