Thanks for all the information and suggestions. Since this non-commercial philosophical forum won't have many new registrations, I won't know for a while if the original problem has been fixed.
I still don't know what happened to the first Registration Activation Request emails. They didn't show up in my regular Member inbox, or in my special Admin inbox. So I guess I'll just have to check-in on the forum more often, to see if someone has tried to register.