First page, my first response. Yes, the message changes according to what HP device/software you have installed, but it's the same issue. These are the latest steps to correct the problem:
A fix was provided rather quickly. Do the following, per fellow forum member, Lawrence Finch:
A. Get the most recent HP drivers, either from:
ftp://ftp.hp.com/pub/softlib/software12/HP_Quick_Start/osx/Installations/Essentials/hp-printer-essentials-S-5_14_8_4.pkg
or
HP Printer Drivers v5.1 for OS X
or
System Preferences > Software Update (if you don't see it, press Command+R)
B. Run the installer.
C. When the installer completes, go to System Preferences > Printers & Scanners, highlight the HP printer and remove it. Then add it back. This is necessary because otherwise it will continue to use the now unsigned drivers and it will appear that the problem has not been fixed. Apparently installing the update does not remove the old drivers.
A note on AirPrint: Your Mac may say it can’t find the AirPrint printer. If that should happen, reboot your router.