/usr/lib/cups/backend/usb does not exist anymore on my Ubuntu 16.04 LTS Xenial Xerus system. I cannot explain why. None of my two printers work anymore because of this.
Anyone can tell me how to create/reinstall this binary?
hplip wasn't installed anymore on my system. Not sure why. I do have automatic updates configured on my Ubuntu 16.04 LTS Xenial Xerus server; maybe there was a problem and the package was dropped.
To fix the problem, I ran:
apt install --reinstall hplip
And now, both printers, HP LaserJet 6L and Brother MFC J615W work again.