Solution: Multifunction scanner problem. Failed to open the connection device (-21355)

After much frustration, I found a solution in a 21 Oct 2020 MacWorld article by Glenn Fleishman:

https://www.macworld.com/article/234654/scanner-wont-work-image-capture-disable-a-network-setting.html

In my case:

Multiple Macs running OS versions High Sierra, Catalina, and two M1 (Air and Mini) on Big Sur could not connect to the scanner in a new HP OfficeJet Pro 9025.  Neither the Mac’s Image Scanner app nor HP’s own Easy Scan could access the scanner, resulting in an error “Failed to open the connection device (-21355).”  Printing was fine.  Scanning on a truly ancient (but UTD w Win 10) laptop went perfectly, showing that the hardware and LAN were fine.

Nothing else I found online helped at all; I was on the verge of declaring defeat when I found Glenn’s article.  Problem solved:

1. Go into your printer’s browser-based setup (eg 192.168.x.x), similar to a home router setup.
2. Enter the security PIN (password) found on a label in the ink cartridge area.
3. NETWORK>General>Network Protocols. By default, “Enable both IPv4 and IPv6” is checked.  Change this to IPv4 only.
4. Save and sign out.

YMMV, especially with other brands/models, but it a super simple fix, once I found it, for me!

Best wishes.