Dear all,
here is the same samsung SCX3400 and M407 printer/scanner multifunction and upgraded to OSX 10.14 Mojave.
I was desperate to fix this problem, but solution was on my computer already. (and maybe on yours too)
1) Search your computer for Scan Assistant.app (it comes with printers software package)
on mine computer it is located here: /Applications/Samsung/Scan Assistant.app
2) Wait for a moment and it will find your printer/scanner.
3) Scan with this utility, and all goes fine.
There is also additional software from samsung: Samsung Easy Printer Manager
It is also has scanning ability and works fine, but need to be installed from samsung site.
Search is not working anymore on that site, here is direct link for latest OSX version I have found: DOWNLOAD
Please respond here if this helps, i want to know.