Content editor is blank on the backend since upgrading to joomla 3.8.1. It works in Joomla articles but not in K2 items. I'm on a mac with os 10.12.6 using both Firefox and Safari. Host is Siteground. K2 is 2.8

Do You have a backup?
Which editor are you using?
Try switching to TinyMC and see if it shows
Try upgrading your current editor from its vendor

Hello,
I am facing exactly the same problem. Did you find a solution?
Joomla! 3.8.7, K2 2.8.0 , PHP version 5.6.36, K2 store 3.8.5 .

The text (intro & fulltext) appears only when I set the editor to none, but of course not everyone is familiar to html code.

Curiously If I set TinyMC doesnt appear the editor at all...

Hello,

This is most likely related to a JS issue.
Can you check under the console tab of your browser's tools for any errors?