[Plugin: Smarty Pants Client Document Manager] CGI Error when signing in

I’m recently installed the SP Client Document Manager plugin. I created two test users and created a test folder and uploaded a test document for each user. I then went to the website from a different browser (one that the admin is not signed in on) and attempted to log in to the document manager (from the page with the short code in it) with one of the test username and password combinations. When I had the “Theme My Login” plugin activated, I got a CGI error saying that something misbehaved and didn’t provide complete HTTP something or other. When I had that plugin deactivated, it simply kept reloading the log in page, instead of actually logging the user in.