Second install of WP as subdomain

I’ve put up a second install of wordpress in a subdirectory so that one of my clients can see a demo for what I’m planning for her site.

Under settings the WP address is set as: http://the-sacred-path.com/testingsub (since it is in a subdirectory off of public_html).

The blog address URL is set as http://testingsub.the-sacred-path.com/

When I click on a page link, or a post title, I get an internal server error:

The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, webmaster@testingsub.the-sacred-path.com and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.

I know I’m probably going to bang my head on the keyboard when I find out what the problem is, but I would sure appreciate someone pointing me in the right direction as I’ve never done a subdomain setup with wordpress.