The Layout.php fix is part of the fix, however there is another portion you should implement regarding the session. You can find the full fix via the Magento StackExchange, note that this answer links to a blog post that fully explains it.
If you have any questions about the solution in that answer as well as the full changes linked there, you can ask here and one of us will likely be able to assist.
Step 2 doesn't seem to help. For Step 2, I did a search and replace in Varien.php for $this->data and replaced with $_SESSION but I still get a 404 Not Found 1 error when I try to access the admin page