Error "Unknown Error" when uploading a file with CKEditor
After digging, i have found that the problem was in the function packages/kcfinder/core/class/uploader.php :: checkFilePath

This function forbids an upload if you have a symbolic link in your path.

There is also a related issue in CiviCRM JIRA : https://issues.civicrm.org/jira/browse/CRM-15607

EDIT: 

Probably not a very good solution but you can patch kcfinder. 

In my case it's complicated to remove the symbolic link so I used this as a temporary fix.

Add the following line as the first line of the function checkFilePath to avoid the verification:

     protected function checkFilePath($file) {
+        return True;
         $rPath = realpath($file);
         if (strtoupper(substr(PHP_OS, 0, 3)) == "WIN")
             $rPath = str_replace("\\", "/", $rPath);
         return (substr($rPath, 0, strlen($this-&gt;typeDir)) === $this-&gt;typeDir);
    }