I have a wordpress site with many menu and sub-menu items, and suddenly, cant add items any more :). I stuck for about 2 hours solving this problem, some totorials advice to modify the php.ini file on the server. In my case, it is a little bit difficult cause I am working on a Azure host.
Finnaly, this code saved my life:
max_input_vars = 5000;
You need to create a file named .user.ini and put the code in it, Then put the file in the root folder of your wordpress site. You will see, it works like a charm !!