Today I was suddenly unable to use my home HP ink jet printer with Linux Mint, despite the fact that this morning it was printing fine from Windows (my system is dual-boot). The error message informed me that /usr/lib/cups/backend/hp had failed for unspecified reasons. Toggling the printer off and back on did not help, nor did rebooting the system, nor did switching the driver.
A quick web search, however, revealed the fact that some unspecified update had screwed up permissions on the /usr/lib/cups/backend directory (restricting it to root only). Most of the executables in that directory looked okay, but the permissions for cups-pdf were also root only, so I'm guessing that it was an update to cups-pdf that blew things up.
The solution: sudo chmod -R 755 /usr/lib/cups/backend.
I thought Microsoft had a patent on destructive updates. Guess not.