Hello,

It works with the default theme because it doesn’t have extra fields. Try to create a menu that has 4 times more menu items and test how it works with the default theme. You will see the same error.
Probably, you didn’t increase the parameter correctly or it is limited by something else on your server. But the fact is that the request can be processed with large amount of data. You can check this yourself by saving a simple menu. It works without any problems.

Kind Regards