Payment not working

Unfortunately, payment on my website does not work at all.
When I fullfil all the details in the cart and press “pay” button, this plugin redirects me to a page, which does not exist and I got 404 error – with no possibility to pay.

Here is a link, which has been generated by the plugin:

https://www.kryptonakup.sk/objednavka//order-pay/2691/?pay_for_order=true&key=wc_order_rQSsNcX9Q2PKR

(Objednavka means order)
There is even a double backslash, so there must be some bug.