Hi there,
I double checked the htaccess file and it seems to be the default WordPress one so this must be a redirection which is related to your environment which we can not access you need to contact your hosting service provider and ask them to help.
I tested by disabling all third-party plugins and changing the theme from the child theme to the parent one. I also changed the main URL of the Builder from x to builder. To do so I went to Pro > Settings.
The problem is originated from the URL which is getting loaded when you click on the templates button:
It should be: https://testrobbin.puppygroep.nl/x/cornerstone-endpoint
but it retrieves: https://testrobbin.puppygroep.nl/cornerstone-endpoint
This problem is presented on none of our installation, you will need to contact your hosting service provider to follow up the case and fix the redirection issue.
Thank you.