Hi Tiffany,
If an invitee starts the registration process and leaves the page without saving it/going the next page, the registration attempt expired after 20 minutes.
That's when they see the Registration Expired message. However, the system will allow them to start the registration process again.
This can also happen due to browser cache. The invitee can clear the browser cache and that should resolve the issue.
Hope this helps!
Regards
Ankita Gupta