400 Bad Request when logging into XProtect Server

SSL verification and base64 username/password seem to be working. What could be causing the 400 Bad Request? Server is Essential+ and the Login .Net SOAP example works when I log into the server as a basic user. My code is attached.

Here’s the server’s response:

HTTP/1.1 400 Bad Request

Cache-Control: private

Server: Microsoft-IIS/10.0

X-AspNet-Version: 4.0.30319

X-Powered-By: ASP.NET

X-Frame-Options: SAMEORIGIN

Date: Fri, 29 Jun 2018 19:09:53 GMT

Connection: close

Content-Length: 0

Instance id must be a GUID.