Hello, I'm having problems with k2 using extra campor and Save new articles.
J3.5.1 and K2 2.7.0 installed. See the image below for an error when trying to save the article.

In other articles is showing the line: Warning: trim () expects parameter 1 to be string, array given Beheerder in /home/****/public_html/new/libraries/vendor/joomla/string/src/phputf8/trim.php on line 61

In Adiministrator some article appears the line: Warning: htmlspecialchars () expects parameter 1 to be string, array given Beheerder in /home/****/public_html/new/administrator/components/com_k2/models/extrafield.php on line 308

Only this site this is happening, using Fields Extras. In other site there are no errors with the K2 2.7.0.

PHP 5.5 enabled on the server.
Thank you.