Checkout gives error 500 and "Not enough information to process payment"

-- url of the page with the problem -- : www.zappi.info
-- HikaShop version -- : 4.0.1
-- Joomla version -- : 3.9.0
-- PHP version -- : 7.2
-- Browser(s) name and version -- : Any browser
-- Error-message(debug-mod must be tuned on) -- : Not enough information to process payment

After checkout an Error 500 shows up, after refreshing an error page from checkout/ CCIdeal plugin shows: Not enough information to process payment (this is the link: zappi.info/shop/producten/checkout/confi...atform&task=bankform

The order is created in Hikashop, in the client account the order is visible. But the payment doesn't begin .

This error came together with the update either of Joomla 3.9.0 or from Hikashop 4.00 - I don't know.

The test payment of CCideal is working fine.

Can any one help me here ?

In error log I get this message:

Fatal error: Cannot declare class Sisow, because the name is already in use in /zappi.info/public_html/libraries/sisow/sisow.php on line 2

I changed over to Mollie, sorry for Sisow ! But now everything works fine again.