White Screen of Death after 3.6.1 upgrade

Just tried to update a client to 3.6.1 an when I did all sorts of issues transpired. I decided to do a complete install but I still get the white screen of death when I’m on the site and I get this message when I go to the wp-admin:

Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, ‘_show_post_preview’ was given in /home/yoursite/public_html/wp/wp-includes/plugin.php on line 406

Line 406 is

call_user_func_array($the_['function'], array_slice($args, 0, (int) $the_['accepted_args']));

So uploading a fresh install didn’t seem to work. What will?

Yes, I backed up my database fortunately.