Callback failure when Force secure checkout HTTPS set

Hi,

I am having a problem when I set the Force secure checkout option to use HTTPS. Everything works fine when I don’t have this option set. I am currently using the test option

I am getting a callback failure alert email as follows:
Our systems have detected that your callback has failed.

This callback failure means we were unable to pass information
to your server about the following transaction:

Error reported: Callback to: https://artsplus.co.uk/wc-api/MDS_Worldpay_Gateway/: failed CAUSED BY Received fatal alert: internal_error
Server Reference:

I believe the SSL certificate is setup OK as the PayPal option is working Ok.

I hope that you can help me
Thanks
Jenny