\Dialogs\SSMNavigator\PrefixImportedSheetTitleWithFileName and the REG_DWORD value was a large odd number. I changed the REG_DWORD value to 1 (does not matter if it is hexadecimal or decimal type). The "Prefix sheet titles with file name" box is now checked and works properly. If this value goes out of whack periodically, you can add a shortcut to it in your Favorites from within the registry editor, so you can easily find it to change it back again. <current profile> in your registry will be the actual name of the profile you have current in your AutoCAD. This fix should work for all versions of AutoCAD.