If something strange happen, like this one, always check httpd errors first.
I got a similar problem in acf field creation area.
[error] [client 127.0.0.1] PHP Warning: Unknown: Input variables exceeded 1000. To increase the limit change max_input_vars in php.ini. in Unknown on line 0, referer: http://local.dev/wp-admin/post.php?post=728&action=edit
Yep, error tells you what to do. Just increase max_input_vars value in your php.ini. I see my local installation dont even have this value but you can just add it to php.ini and restart server. It will work 🙂
Some discussion on this: http://stackoverflow.com/questions/10303714/php-max-input-vars
