Hi Alberto,
Sorry to hear that you ran into this issue.
I tested embed blocks with the "Block Entrance Animations" option enabled and my course previewed without any issues. Here's a link to my test course. Would you mind replacing your embed code with this one to see if we can isolate it to the iframe code you're using for embedding content?
<iframe width="560" height="315" src="https://www.youtube.com/embed/EbKAqjbVE4s?si=vAv-YG3yZKDXu2EN" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>
If the issue is isolated to your iframe embed code, please post it here so we can check to see if we can modify it to get it to work properly.
