CGI Error when Client Attemps Login

I’ve 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 as a client (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.

https://wordpress.org/plugins/sp-client-document-manager/