You are referring to Appearance > Menus?
It works perfectly on my development site. So it seems to be specific to your site. Could you email me at mollie@paytium.nl so we can discuss further?
The issue seems to be caused by “Conditional Payment Methods for WooCommerce”, which used a javascript function or library that causes the error “”uncaught reference error columns is not defined”. The Mollie plugin doesn’t use it, but because that other plugin handles payment plugins it looks like the Mollie plugin was causing the issue.
It is solved, indeed it wasn’t the Mollie plugin but the “Conditional Payment Methods for WooCommerce” plugin. I deactivated that and now menu management works again. Thanks again Davdebcom!