I spent two hours like a mad man following solutions I found on the web regarding 500 error all over the site after upgrading. I did a 'manual' upgrade, restored the database, disabled all plugins, used a default .htaccess etc etc, but in the end the problem was this line in wp-config.php:
define('WP_CACHE', true); //Added by WP-Cache Manager
I commented it out and everything is back to normal:
//define('WP_CACHE', true); //Added by WP-Cache Manager
Hope this helps some one who's having the same problem.