The image manager, document manager, etc will not list files or allow upload when logged in as a SuperUser. Without changing a thing, I can login with a non-superuser account, and it works correctly.
 
A general exception is logged that looks like this:
Message: dnnWerk.Providers.RadEditorProvider.FileManagerException: /Portals/0/radeditor/images ---> System.ArgumentException: An item with the same key has already been added. at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource) at System.Collections.Generic.Dictionary2.Insert(TKey key, TValue value, Boolean add) at System.Collections.Generic.Dictionary2.Add(TKey key, TValue value) at dnnWerk.Providers.RadEditorProvider.FileSystemValidation.get_UserFolders() at dnnWerk.Providers.RadEditorProvider.FileSystemValidation.GetUserFolder(String path) at dnnWerk.Providers.RadEditorProvider.TelerikFileBrowserProvider.AddChildDirectoriesToList(DirectoryItem[]& radDirectories, Boolean recursive, Boolean loadFiles) at dnnWerk.Providers.RadEditorProvider.TelerikFileBrowserProvider.GetDirectoryItemWithDNNPermissions(String path, Boolean loadFiles) at dnnWerk.Providers.RadEditorProvider.TelerikFileBrowserProvider.ResolveRootDirectoryAsTree(String path) --- End of inner exception stack trace ---