Comment 13 for bug 1442734

 I downloaded both hplip-3.15.2-plugin.run and hplip-3.15.2-plugin.run.asc from https://www.openprinting.org/download/printdriver/auxfiles/HP/plugins/. I put them both on the desktop (but could be anywhere). I opened HP Device manager, clicked "Install Required Plugin", chose "Select and install an existing local copy of the plug-in file" and agreed to the license terms.

Now I can scan again!