Woocommerce Pace Order returns Internal Server Error

I have read the Must Read articles, and am still having problems.

I have Kallyas Theme and all the latest versions of plug-ins for woocommerce installed, and everything works through to checkout, but when Place Order is clicked, an internal server error comes up.

I am also using Kashflow for Woocommerce by Devicesoftware.com (Switch I think is the developer on WP), and this plug-in is no longer available. I have deactivated it, but this still does not help. If there is a patch for this plug-in, this would possibly help? Switch are you around?

My site is https://www.icanplaytennis.net I am not that good at coding, so simply steps would be appreciated to help me out!

Many thanks everyone
Matt