ChangeYour Theme Completely. Wordpress default dashboard.

I am using a Divi child theme. I have installed a few different themes. When I go into the default WordPress dashboard and click on the 'change theme completely' and choose my chosen theme in the preview mode, I get to see the title, top navigation and then the Homepage is showing as a coded language. It doesn't seem to matter which theme I preview, the home page is always showing as a code.
I have created a test site as a user would do and the same shows in a test site too. Not good for a potential user.
Can anyone help out on this frustrating matter please?

Wishing you all a happy and successful 2016

Peter