Hello @KristinaP68 , You should be able to put in multiple videos without issue. I am sorry that you're getting the rainbow thumbnail. I did pop into your account and was able to get a couple of different Vimeo thumbnails to show. Check the most recent draft that starts with the word "VIDEO". (You can decide which one you prefer and delete the other. )
How I got the thumbnail to work was by going to the video on Vimeo clicking the paper plane icon to get the share link. Then I grabbed the URL out of the embed code section. And that gave me an image thumbnail rather than the rainbow.
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