Issues integrating External Preview App into ContentCreator

Hi community,

I have issues integrating External Preview App into ContentCreator

Setup of DEV env:
FS v. 2025.7, ContentCreator running on https://retailportal-redaktion-dev.i.mercedes-benz.com/
External Preview App running on https://rp-next-dev.app.corpintra.net/preview
ContentCreator configured to use this external preview url
External Preview App includes the live.js, but from its own assets folder. It includes also live.css and colors.css file. This was necessary due to security restriction, we could not reach live.js file from within the iframe.
Connecting via SNAP API works fine
Navigation via onRequestPreviewElement works fine.
data-preview-id are set like this so far:

I would expect:
to see the highlighted border around the article and buttons in top right corner
to see the highlighted border around the picture and edit button in bottom right corner
to be able to edit both headline and picture with Ctrl + left click

What happens:
I see only the highlighted border around the article, but no buttons
I see nothing around the picture
When doing Ctrl + left click on headline or picture there is a short flash and then I get this error in console

Is there something more I need to do? Is there something missing in the configuration?

Thanks for any advice

Best Regards, 

Martin