Upgrade to 3.17.9 causes filter failure

[Big Al]

I foolishly upgraded to 3.17.9 and since then all printing has failed. I downgraded back to 3.16.11 and still printing fails. I reinstalled all the hplip packages (I am running Mint 18) and ran hp-check (log attached) which showed a "filter failed" error. Googling around I found suggestions this was a permissions error, which was confirm by the following cups log entry:
D [20/Sep/2017:18:16:03 +0100] [Job 313] Started filter /usr/lib/cups/filter/pstops (PID 27491)
D [20/Sep/2017:18:16:03 +0100] [Job 313] Started filter /usr/lib/cups/filter/hpps (PID 27492)
D [20/Sep/2017:18:16:03 +0100] [Job 313] Started backend /usr/lib/cups/backend/hp (PID 27493)
D [20/Sep/2017:18:16:03 +0100] [Job 313] \033[31;01merror: Failed to create /var/spool/cups/tmp/.hplip\033[0m

Googling around suggested this was a permission error, but the permissions match what was suggested:
 # ls -lar /var/spool/cups/tmp/.hplip
total 0
-rw-rw-rw- 1 lp lp 0 Sep 20 18:39 hplip.conf
drwxrwx--T 1 root lp 66 Sep 20 18:56 ..
drwxr-xr-x 1 lp lp 20 Sep 20 18:39 .

I am pulling my hair out on this. Printing used to work before the upgrade broke everything.

Incidentally, I tried the upgrade again which takes absolutely ages because at one stage it was doing a "find /home -name .hplip", followed by attempting to delete anything it found. This is a BAD idea without restricting the depth of the search. For example I am running btrfs with snapshots on /home (i.e. the "rm -rf ..." that results will wail on the .snapshots as they are read-only mounts), and my /home is over 30TB with millions of files and directories. I strongly suggest limiting with -maxdepth 1 to restrict the search to the root of people's home directories - assuming that is all you are attempting to achieve.