After checkout being sent to 500_SHTML Page

[Fusionhost]

Hi,

I am receiving a strange error at the moment.

So,
I have completed a order though the hostbill order page and I have selected payment via Bank Transfer.

Upon clicking 'Checkout' to complete the order should of sent me straight though to the invoice but instead it sent me to the blank page which displays the message 500_SHTML; see screenshot.

I have checked the error logs:

02/02/2016 17:37:30   PHP Error   
PHP [Error]: Call to undefined function imagecreatefrompng() IN includes/libs/kcaptcha/captcha.php(68)


So I removed the captcha security and it still returns me to the blank "500_SHTML" page which on further investigation seems to be linked to ioncube.

http://wiki.hostbillapp.com/index.php?title=Fix_blank_pages_issues

---

The order is created within Hostbill and is charged £37.62 but it is always charged at £0.00; payment via Bank Transfer.

Any ideas?

Kind Regards,
Jon