Same issue here.
A user reported this error to me 13 days ago and I thought it was an error specific to this user.
I changed this email by following this guide:
Today another user reported the same issue. So I think there is a more general problem with the email change.
Installation details:
- Discourse 2.4.0.beta9
- Droplet in Digital Ocean with 2GB RAM, 25 GB SSD.
Thanks in advance.