Hey there! I took a look at your video and I noticed this in the Share settings on Vimeo:
"The owner of this video has set domain restrictions and you will not be able to embed it on all websites. Please contact the owner if you would like permission to embed this video"
These domain restrictions may be prohibiting our process for generating the thumbnail. I suggest updating the settings to lift those domain restrictions, then clearing your browser's cache and cookies, restarting the browser, and trying again. Here are some instructions from Vimeo for updating those settings: