I had this same exact issue,
It was resolved quickly by going to Interfaces---->Hotspot Interface----> change ARP to reply-only.
Fixed the issue. All clients are now getting DHCP leases.

What is the resolution to this?

I am having the same issue. New accounts created through the signup page are unable to login, and radius reports user not found, however adding a user account in the admin interface of user manager works, and they are able to login with the manually created account.