After update Joomla! 3.6.4, I can't edit K2 Item, modules, articles... nothing. ERROR DISPLAY: An error has occurred. 0 Error decoding JSON data: Control character error, possibly incorrectly encoded
Do You mean You cannot edit them in joomla panel or in frontend editor ? If it's related with joomla panel try clearing browser/joomla/server cache and if this will not make any difference contact joomla developers.
If You mean frontend editor please send me an access to joomla panel.