Blank checkout page instead of order received page

After a checkout i get for example:
https://dab-webshop.nl/afrekenen/?key=wc_order_55d676a2c835f&utm_nooverride=1

What gateway is it?

Where can I find that? I’m not so familiar with WordPress.

Thanks!

Payment gateway. What are you using to take payment.

I am using a woocommerce plugin from mollie. (Mollie.com)

I think its redirecting wrong. Missing order ID. Needs to use our methods: https://github.com/woothemes/woocommerce/blob/5ef335b169ff4e19a4c5b393963a369446922b0c/includes/gateways/cheque/class-wc-gateway-cheque.php#L127

So what am I supposed to do?
Thanks

Check with gateway developer. If its open to download somewhere I can take a quick look, but cannot patch it.

You can download it here for free:
https://www.ads-software.com/plugins/mollie-payments-for-woocommerce/

This plugin is using the return URL:

$return_url = add_query_arg(array(
            'order_id'       => $order->id,
            'key'            => $order->order_key,
        ), $return_url);

The WC return URL is different https://github.com/woothemes/woocommerce/blob/63fe4a0c0528635a72432b167e60cfd70fd619f4/includes/abstracts/abstract-wc-payment-gateway.php#L93-L105

Please report to the author.