Unfortunately I have the same error, but just to make sure:
before this addition I only had the following in my apiserver.conf

auth {
    # Fixed users login credentials
    # No other user will be able to login
    fixed_users {
        enabled: true
        pass_hashed: true
        users: [
            {
                username: "user.name"
                password: "blabla"
                name: "User Name"
            },
        ]
    }
}

and now after the addition:

auth {
    # Fixed users login credentials
    # No other user will be able to login
    fixed_users {
        enabled: true
        pass_hashed: true
        users: [
            {
                username: "user.name"
                password: "blabla"
                name: "User Name"
            },
        ]
    }
    cookies {
    httponly: true
    secure: true
    domain: ".my.domain"
    max_age: 99999999999
  }
}

Now I noticed in this conf example that all is wrapped under a top {} which is not what I have.
So I can add those top {} , but before I do it I want to make sure my users will not be overrided or recreated..