hi bartkphoto,
If there is a same issue while activating the default WordPress theme or any other theme then its not the issue of our theme. However, we may help you solve this issue what you need to do is just provide us the temporary login credential of your admin site as well as your ftp login detail if you feel secure via email. There might be something issue with your host or anything else.
Also we have quick research into your site and found that something was injected into your site and trying to redirect to the different site.
Do not share any login credential in this forum because this forum is public and visible to all.
Thank you!