The cache needs to ignore including PHPSESSID as part as the tmp cache file name because this is creating duplicate cache files.
title-index.php-PHPSESSID-ac48d484c831300da3feb2c7fc73265b.tmp
......................^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
A quick fix for now is to add this at top of head.php:
ini_set('session.use_trans_sid', false);
ini_set('session.use_only_cookies', true);
ini_set('url_rewriter.tags', '');
