@Dimiikou you helped allot! I didn't exactly do your suggestion but I took a closer look at the encoding.
I changed the encoding from
base64EncodedAuthenticationString = Convert.ToBase64String(System.Text.ASCIIEncoding.ASCII.GetBytes(authenticationString));
to
base64EncodedAuthenticationString = Convert.ToBase64String(System.Text.Encoding.ASCII.GetBytes(authenticationString));
and it worked!!!