Please bear in mind that with the addition of the “WordPress user integration” feature that lets you integrate the members of eMember witht wordpress users. Basically if you use this feature then the plugin will automatically create a new wordpress user with the same credentials when a new member is registered with eMember. This way the members only have one set of login credentials. Some people like to use this feature some don’t. I personally don’t like it cause I don’t want my members to be a user of my WordPress installation too… I like to keep them separate.
With that said, WordPress requires the members to have unique email and username. So we had to make the system to not accept emails or usernames that have already been taken.
Can you please verify by looking at your members list from the “members” menu of the plugin to make sure that the email address you are using is not already being used by one of the existing members? Because you are using sandbox testing, I have a feeling that you have already ran this once so that email is already registered in the system.