Error Deserializing Xml to Object - xmlns='' was not expected

Simply take off the Namespace =:

[XmlRoot("register-account"), XmlType("register-account")]
public class RegisterAccountResponse {...}

since your xml doesn't seem to be in an xml-namespace. Also, [Serializable] isn't used by XmlSerializer.

If your xml was using a namespace it would have an xmlns at the root.

Also, to help with callers you could add where T : class, new() (the , new() being the addition) to your Deserialize method, since XmlSerializer demands a public parameterless constructor.

Nothing worked here for me

What worked is to MAKE SURE that the C# Class (main class) you are trying to map/deserialize the xml string to HAS AN XmlRootAttribute that matches the root element of the response.

Check my full answer with an exmaple https://stackoverflow.com/a/61525536/1594274