Script Installation Issue on Ghost Blog

Hi,
I'm using ghost for blogging and tried to install the script manually. Unfortunately I keep getting the error message We're Having Trouble Seeing Your Script.
What I've tried:
- Uninstalling all browser plugins
- Clearing the cache
- Removing all other scripts which could be potentially blocking from my header.
- Installing via Google Tag Manager instead of placing the script in the header.
Nothing worked. I also contacted the Ghost support who confirmed that the script has to be placed in the header of the custom code section.
Can you please assist? Thank you in advance.
Lea