I have the same problem on a couple of sites whilst trying to update JCE.
When I tried to update it says: Error updating package.
Now the joomla back end shows an error message: Class 'WFApplication' not found
If I try and install manually it says: Joomla\Filesystem\File::delete: Failed deleting inaccessible file editor_plugin.js
Other plug ins updated correctly and permissions are set to 755 / 644. I've also checked the ownership is correct.