When you remove the XmlMembershipProvider attribute, are you replacing it with another "defaultProvider"?
Does the "editor" account exist in your SQL DB? The 2 uncommented providers in your <membershipSection> are XmlMembershipProvider and AspNetSqlMembershipProvider. I'm assuming that the 2nd one (SQL) is kicking in when you remove XmlMembershipProvider.
You might want to check and see what users are defined in your SQL DB. It may be that an editor is defined, but an admin is not defined.
BE has an "Administrators" role. So the most important thing is that one of the users in your SQL DB is tied to the "Administrators" role. If you have that setup, then you should be able to log in under that user.
You might want to add the Administrators role to the "editor" since you are able to log in under the editor. After making the editor an administrator, you could then go in there and create a real administrator account.
If you make any changes directly to the SQL DB, you may need to restart the application in order for BE to see the latest data (since it often caches data in memory). You can restart the application by making any change to the web.config file (add
a space, etc).