Upon trying to open the subdomain in my browser, I expected to (maybe) find a Login window first, and then something like what you suggested.

Well, i assumed that was already working ;)

Do you have access to the apache logs? Or if you're not using apache, the logs of your httpd?

They might point you to the cuase of the problem.

There is a couple of threads in this forum about HTTP 500 erros, have you read them?

Did you set all file permissions properly?