Can't open files backed up to an external hard drive.

"Using Backup (Windows7)..."

That was unfortunate. Backup & Restore (Windows 7) is not supported for making backups. It was included in Windows 11 (and Windows 10) so that people who used it to make backups in Windows 7 and Windows 8 are able to restore their backups in Windows 10 and Windows 11. Nowhere on Microsoft's website will you read that Backup & Restore (Windows 7) is approved for making new backups.

For you that means there are no official answers. You either have to figure out something on your own (perhaps with assistance from artificial intelligence) or someone here (or elsewhere) has a tip to share.

The message means that the .xml file (which describes the file structure of the backup) is unreadable.

I reviewed your screenshot and noticed something important: the error is occurring when your browser attempts to open an XML file inside the WindowsImageBackup folder, rather than one of your actual backed-up documents. The message ERR_ACCESS_DENIED indicates that access to this file was denied, but it does not necessarily mean the XML file or your backup is corrupted. These XML files contain backup-related information and are not intended to be opened to retrieve your personal documents. If you created a file backup, first try opening Control Panel > Backup and Restore (Windows 7) > Select another backup to restore files from, select your external drive, and follow the restoration wizard.

https://support.microsoft.com/windows/deployment/install-upgrade/find-lost-files-after-upgrading-windows

However, the WindowsImageBackup folder shown in your screenshot suggests you may have created a system image rather than an ordinary file backup. Open File Explorer, navigate to the WindowsImageBackup folder on your external drive, and look inside the dated backup folder for files ending in .vhd or .vhdx.