Can't create Realm "Realm name xxx is not allowed"

When I try to create a new Realm I get the error message "Realm name xxx is not allowed" with the default name or with something really simple.

I have seen the similar bug reports about problems renewing Realms and checked the account permissions as advised but it doesn't resolve the issue. I have selected 'allow' for absolutely everything on the account but still get the error. The account is a child account linked to my adult one.

I have tried logging in on iPhone, iPad and Android and got the same error on every device.