I am getting this error that I never got before. We just moved the site to a different server - both hosted with GoDaddy and using CPanel. Both sites are on PHP 7.0. Both sites are on Joomla 3.9.8. Both sites are on K2 2.9.0.
But on the new server, I get the Call to undefined error. I found another item in the forum about this but it was unrelated to my issue. But it still lead me to the fact that it could be a finder issue. I went into "plugins/finder/k2" and renamed this folder and the error went away. So then I re-uploaded the folder in case something happened with the transfer process. Still get the error. I've re-installed K2 and that didn't help. So I'm not sure if this is a PHP or Server issue. But why would changing that k2 finder folder fix the error and everything seems to be working properly. Is that folder even needed? If it is, what else could go wrong with that disabled?
Thanks for any help you can provide.

This was the article that I found. But they were on PHP 7.1 AND when I tried the disable the com_finder that didn't work. Only disabling the k2 finder helped.
www.joomlaworks.net/forum/k2-en/47847-error-on-save-with-k2-and-php-7-1-x

I may have found the problem. The iconv PHP Extension is not installed, however the server only makes this available for 7.1 and up. The site won't work on PHP 7.1 because it's on Gantry 4 and upgrading to Gantry 5 would be tough because of how customized the site is. So not sure what to do.

Sorry for all the quick posts but I'm hoping this helps someone else. I was able to install iconv through Terminal on the server and now it's working properly. So if you are on PHP 7.0 and having this problem, look to see if iconv is installed in your PHP extensions.