UserGroup.GetUserInfo returning "Exception of type 'Microsoft.Sharepoint.SoapServerException'" (VB.NET)

Hi,

Using the method GetUserInfo from the UserGroup web service (/_vti_bin/usergroup.asmx), I'm getting the following error:

"Exception of type 'Microsoft.Sharepoint.SoapServerException'"

The problem occurs in Sharepoint 2013 Foundation, using FBA Authentication.

 Dim auth As New Authentication()
        Dim UsrGrp As New UserGroup()
        auth.CookieContainer = New CookieContainer()

        Dim LoginResult As LoginResult = auth.Login(pUserSP, pPwdUserSP) 'admin user from sharepoint.

        If LoginResult.ErrorCode = LoginErrorCode.NoError Then
            Dim cookies As CookieCollection = auth.CookieContainer.GetCookies(New Uri(auth.Url))
            Dim authCookie As Cookie = cookies(LoginResult.CookieName)

            Dim webUrl As String = pEnderecoPortal
            Dim copyService As New Copy()
            copyService.Url = webUrl & Convert.ToString("/_vti_bin/copy.asmx")

            copyService.CookieContainer = New CookieContainer()
            copyService.CookieContainer.Add(authCookie)



            UsrGrp.Url = webUrl & Convert.ToString("/_vti_bin/usergroup.asmx")
            UsrGrp.CookieContainer = New CookieContainer
            UsrGrp.CookieContainer.Add(authCookie)
            Dim userId As String = ""
            Dim xmlResult As Xml.XmlNode
            Try

                xmlResult = UsrGrp.GetUserInfo(pCondomino) '<--- ERROR OCCURS HERE. pCondomino contains a valid username ("admin").

                userid = xmlResult.ChildNodes(0).Value("Id").ToString()
            Catch ex As Protocols.SoapException
                MsgBox(ex.Message)
            End Try

The code is running against a remote sharepoint server. I can't remote debug and I can't find anything on the server (eventvwr shows nothing).

Thanks, guys.

J.