Add instruction to fix TailsData_unlocked ownership

wiki/src/support/known_issues.mdwn

@@ -590,6 +590,14 @@ If you encounter this problem:
 Your <span class="filename">Persistent</span> folder and persistent feature
 configurations should be restored.
 
+If the above instructions do not fix the problem, you might need to fix the
+ownership of TailsData_unlocked.
+
+To fix the ownership of TailsData_unlocked, set an administration password and
+execute the following command in a terminal:
+
+`sudo chown root:root /live/persistence/TailsData_unlocked`
+
 <a id="restart-shell"></a>
 
 ## Icons and information located on the top right corner of the screen disappeared