Getting domain authentication error when embedding Marketing forms to an external landing page

Recently, I have encountered an issue in Dynamics 365 Marketing where I was not able to embed the marketing form, I created to an external landing page.

1. I created Marketing form and published it.
2. I whitelisted the domain and these domains were authenticated.
3. I added an associated marketing page to the marketing form.
4. I copied and pasted the code snippet to paste in to the external landing page.

I got the following error.

It left to me surprise that why I am getting such error when I have authenticated domain. I researched about it and all the blogs; forums referred that

1. Add an authenticated domain to your Marketing instance.
2. Remove Chrome extensions and re-paste the code snippet.

But none of them worked. So, I tried following. I added storage.googleapis.com as an authenticated domain and whitelisted it in the Marketing forma and it worked. Erueka!!!!