The only thing I can think of is either media applied to templates or perhaps something in the Announcements Library. If any of your templates are not displaying a BG when you are looking at the list, it could be the issue. If you then apply the template, it should tell you it is missing the media.
If it is something in the Announcement Library, replacing it with a blank one will let you know. Attached is a clean file. With MediaShout closed, go to C:\ProgramData\MediaComplete\MediaShout 6\Announcement Library. Rename the one that's there (don't delete it) and place a copy of the one I attached.