I have an Epson ET-2850 printer. When I try to scan I get this error message: Failed to open a connection to the device (-21345). Please advise.

Have a Epson ET 2750 which was used in Big Sur and again in Monterey in the latest stable version of Monterey 12.5.1

You may need the Epson ET-2850 Scanner Driver and Epson Scan 2 Utility v6.4.99.

https://epson.ca/Support/Printers/All-In-Ones/ET-Series/Epson-ET-2850/s/SPT_C11CJ63202

Hi, there are a couple of things that you can try.

1. Disable the firewall on the Mac and try scanning again - if it works, you will need to add the scan / print application to the firewall exclusions.
2. Disable IPv6 if you are not using it.
3. If you are trying to scan whilst connected to a VPN - close down the VPN tunnel and try.

You are welcome and thank you too 🇨🇦

Do come back to the Apple Support Communities ( ASC )  in the future, if / or when additional questions may arise