HI,
Not sure if this will help you, but we had a similiar problem, after the upgrade you could access the actual website without a problem, but if you tried to access the BO it gave an error.
We got around this by making sure the site is not in Debug mode.
Edit /config/defines.inc.php - make sure the debug part is like below
/* Debug only */
if (!defined('_PS_MODE_DEV_')) {
define('_PS_MODE_DEV_', false);
Then delete the CACHE directory in the /VAR directory
Then try to access the BO again
Worked for us