Checkout page doesn't work after Upgrade 4.0.3

-- url of the page with the problem -- : www.meintraining.at
-- HikaShop version -- : 4.0.3
-- Joomla version -- : 3.9.5
-- PHP version -- : 7.3.3
-- Browser(s) name and version -- : Safari 12.1
-- Error-message(debug-mod must be tuned on) -- : no error - checkout page looks weird and checkout doesnt work

Hello, we upgraded to Hikashop 4.0.3
now we noticed that the checkout page looks very weird and doesnt't work

there is menu entry with type: checkout (also tried the legacy-mode checkout)

the checkout page is: www.meintraining.at/shop/checkout
it looks weird, a white blank area, and below the login/register/guest part
when I change the selection, the console says: ReferenceError: Can't find variable: displayRegistration, onchange checkout :27

Please help, webshop no longer working .. !
I sent login details already for ticket
www.hikashop.com/support/forum/discount-...-floating-taxes.html

when I set the default template, it happens the same and adds message at bottom:
#0 Cannot pass parameter 3 by reference

Hello - I think I found it : we still had the legacy setting set to YES
when I set to NO it works again...