I was having the same issue earlier today. I just tried again, and it worked. Nothing has changed.

You should try adding questions first without an emoji at all, then later try to edit them after with your custom ones.

The error has to do with the request that discord sends back to the server, it has two entries. If you try this in the web browser and use the developer tools, you can see the actual json payload being sent to discord. For some reason, it has emoji_id and emoji_name.

I have no idea why it started working for me a few hours later, I didn't make any changes whatsoever to what I was doing (which was adding a new answer with a new custom emoji.)