Hi.
I know your problem come from “Mega Menu”, but the problem of “Mega menu” come from max_input_vars. When you saving the menu with mega menu feature is activated, the website post many variables to the server. The variables include your menu items & all options for each menu item. So the number of variables can up to thousands. We need to increase the max_input_vars on server so the server can received all variables and save to database.
I checked your server but nothing change. Please check the attachment file.
Thanks.