There's not likely anything wrong with the file mentioned in the error message.
It's telling you that the "sessions" infrastructure of your PHP setup isn't working right -- likely because the "cache" folder on your site isn't set with read-and-write permissions.
What are the permissions on your cache folder ?
What's set for the following entries in the bottom of your /includes/configure.php file?
Code:
define('STORE_SESSIONS', '????');
define('SQL_CACHE_METHOD', '????');
define('DIR_FS_SQL_CACHE', '???????????');