Are you getting this error?
This error is mostly caused when a WordPress automatic update failed in middle for some reason.Manually updating WordPress via ftp will solve the problem.
Fatal error: Call to undefined function is_blog_installed() in /home/xxxx/public_html/newsite/wp-includes/load.php on line 447 Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'wp_ob_end_flush_all' not found or invalid function name in /home/xxxx/public_html/newsite/wp-includes/plugin.php on line 429
Well, I got this error in one of WordPress site I am webmaster at.
I have checked the error log and found this error message.
PHP Fatal error: Call to undefined function is_blog_installed() in /home/xxxx/public_html/newsite/wp-includes/load.php on line 447
Reason of this Error Message..
Once I checked the files I found out that the WordPress site was using WordPress 3.7 .Automatic update to WP 3.8.1 was taken place there and due to some file permission issue the automatic update not worked out.
Due to this error I am not able to logged in to admin panel. I have downloaded WP 3.8.1 and updated the site manually.
This may be not the reason of your error.So try to deactivate themes and Plugins one by one.You will find out the plugin or Theme causing the error.