I had this reproduce with 2 users today, so I'm assuming this is fairly straight forward to reproduce.
1. Add some users and generate welcome emails
2. Have those users be really lazy and not click on the link in the email for a while.
3. Once they do they're given a message about how the link has expired and they need to request a new one.
4. Clicking the only button on the page results in 404 error.