"Failed to open a connection to the device (-21345)" on one computer

Hello quokka,

Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you're having issues with your Scanner due to an error. It looks like you've taken excellent steps so far! Try the steps listed here to see if they resolve the issues with scanning: Solve scanning problems on Mac - Apple Support

• "Make sure the scanner is plugged into an electrical outlet and turned on, and that its cables are securely connected.
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• If you previously saw a scanning option and you don’t see one anymore, try deleting the scanner (or multifunction printer) and adding it again.
• If you’re pressing a button on your scanner to perform the scan, try using an app instead, such as Image Capture or Preview, or go to the scanner manufacturer’s website to see what other apps are recommended. See Scan an image using Image Capture.
• If the scanner is connected to the USB port on an AirPort base station, connect the scanner to a USB port on your Mac.
• If more than one user is logged in to your Mac, log out every user. Then log in again and try to scan."

We hope this helps.

Cheers!

The other thing to try is disable IPv6.

Try disabling the firewall on the Mac - if the scanner then works you will need to add the printing application to the exceptions.