‘username can’t be blank’ when setting up Zoom webinar topic

I wonder whether you are getting an email back for the user from Zoom at all. Maybe the API key does not have the correct scopes set?

This is the API call that I think is not returning data: https://marketplace.zoom.us/docs/api-reference/zoom-api/users/user (or maybe your user’s status is pending?)

Yes, confirmed. There might be a fix coming for this soon, for the moment, you can go to the deleted topic and from the topic’s admin menu you can click on Remove Webinar.