This error occurs because your server limits the amount of data that can be saved at one time. There are two ways to increase this limit:
1) Edit your php.ini file and increase the max_input_vars function to around 3000 e.g.
max_input_vars = 3000;
2) Alternatively edit your .htaccess file (in the root of your WordPress installation) and add:
php_value max_input_vars 3000
If you're not able to do either of these please ask your webhost to do this for you.