I’m having a similiar problem related to the upgrade.
I could not get the upgrade to 4.1 to work via WordPress’s automatic feature, doing this I got the error that everyone above has been commenting on.
Thus I went in through my hosting account. There the upgrade appeared to work successfully, but I get this error now when trying to log in:
Fatal error: Call to undefined function wp_json_encode() in /home/content/29/6593929/html/wp-admin/includes/misc.php on line 668
I can’t get to my dashboard?
