Updated October 8th, 2020.
Mac error -9923 can occur under some conditions when scanning over a network. If it does, you might get a message saying “An error occurred when communicating with the scanner.” The result: you can’t scan.
The problem seems to pop up out of nowhere. And printing will continue to work. It’s only scanning that’s an issue. Weird.
If you do a Google search for this issue you will find many totally unhelpful tech “support” responses from HP and Epson (Canon is better). They tell you to uninstall drivers, reinstall drivers, turn the printer/scanner on, turn the printer/scanner off, restart the router, restart your Mac, restart the coffee pot. Total waste of time.
The real answer is in the network settings. You have a choice: adjust the printer’s network settings (harder, but it solves it for the whole office), or adjust your Mac’s settings (super easy, but it only solves the problem for you).
Solving the problem by adjusting the printer’s network settings amounts to turning OFF IPv6 on the printer. This is done various ways, depending on the printer. With some printers, you’ll see a panel on the front, and you’ll go to the Settings, and then Network (or maybe LAN settings), and eventually you will find a place to turn IPv6 off. With other printers, you’ll type the printer’s IP address into a browser, and that will give you control over the printer’s settings. Some printers come with their own Mac app, which you can use to adjust settings. Either way, it all comes down to one thing: you want to turn OFF IPv6 on your printer. (This assumes that IPv6 is off on your Mac. The idea is we want the same settings– on, or off– on both the printer and the Mac.)
The easier way: fix it at the Mac end. Go to Apple menu / System Preferences / Network, then click the Advanced button.
After you click Advanced, click TCP/IP (at the top). Then change IPv6 to “Automatically.” Then click OK, then click Apply. Then close the System Preferences window.
If IPv6 already said “Automatically” try changing it to Link-local, then clicking OK, then clicking Apply. Then go back in and change IPv6 to “Automatically” again.
If I were you, I’d try changing my Mac’s Network settings first. It’s easy, and easily undone. If it works, great– you’re done. If it doesn’t work, put things back the way they were, and then tackle the problem from the scanner/printer’s end, by changing the printer to disable IPv6.
As you can see, both methods try to match up the Mac’s network settings with the printer’s. In the first case, we change the printer’s settings so it matches the Mac’s. In the second case (the one I’m recommending), we’re changing the Mac’s setting to match the printer’s. Regardless of the method, the proof is in the scanning: if, after changing the Mac’s settings, you can scan, voilá! If you can’t scan, change the printer’s settings (in the printer itself). That should do it.