I found this doc User Guide for Cisco Digital Media Player Device Manager 5.4.x - DMP Access and Security Settings, which may help you regain access.
If that doesn't work, you may need to resort to remote commands. This discussion explains how to do that:
URL to send remote control commands to a DMP
Finally, if you do finally get access to edit the HTTP, remember to use the !DOCTYPE element, as well as meta fields. They might look something like this:
<meta content="text/html;charset=utf-8" http-equiv="Content-Type">
<meta content="utf-8" http-equiv="encoding">
I hope this help!