/etc/hp is empty after installing hplip package - 20.04LTS

[Allan W. Macdonald]

After upgrading to 20.04LTS, my HP-LaserJet 1018 printer stopped printing. Looking at the logs, I found the following errors:
E [04/Apr/2021:12:12:55] common/utils.c 177: validate_plugin_version() Plugin version[3.17.10] mismatch with HPLIP version[3.20.3]
E [04/Apr/2021:12:12:55] common/utils.c 210: Plugin version is not matching

I tried completely purging and re-installing the hplip package and the problem still exists.

I then tried installing the correct plugin - here is what happened:

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$ sudo apt update
Hit:1 http://ca.archive.ubuntu.com/ubuntu focal InRelease
Hit:2 http://ca.archive.ubuntu.com/ubuntu focal-updates InRelease
Hit:3 http://ca.archive.ubuntu.com/ubuntu focal-backports InRelease
Get:4 http://ppa.launchpad.net/system76-dev/stable/ubuntu focal InRelease [17.5 kB]
Hit:5 http://security.ubuntu.com/ubuntu focal-security InRelease
Fetched 17.5 kB in 1s (22.0 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
$ sudo apt install hplip --reinstall
Reading package lists... Done
Building dependency tree
Reading state information... Done
0 upgraded, 0 newly installed, 1 reinstalled, 0 to remove and 0 not upgraded.
Need to get 204 kB of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 http://ca.archive.ubuntu.com/ubuntu focal/main amd64 hplip amd64 3.20.3+dfsg0-2 [204 kB]
Fetched 204 kB in 0s (781 kB/s)
(Reading database ... 303799 files and directories currently installed.)
Preparing to unpack .../hplip_3.20.3+dfsg0-2_amd64.deb ...
Unpacking hplip (3.20.3+dfsg0-2) over (3.20.3+dfsg0-2) ...
Setting up hplip (3.20.3+dfsg0-2) ...
Creating/updating hplip user account...
Can't open /etc/hp/hplip.conf: No such file or directory.
Can't open /etc/hp/hplip.conf: No such file or directory.
Processing triggers for dbus (1.12.16-2ubuntu2.1) ...
Processing triggers for man-db (2.9.1-1) ...
$ wget https://developers.hp.com/sites/default/files/hplip-3.20.3-plugin.run
--2021-04-15 08:51:22-- https://developers.hp.com/sites/default/files/hplip-3.20.3-plugin.run
Resolving developers.hp.com (developers.hp.com)... 35.167.236.162, 34.210.182.99
Connecting to developers.hp.com (developers.hp.com)|35.167.236.162|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 11514166 (11M)
Saving to: ‘hplip-3.20.3-plugin.run.2’

hplip-3.20.3-plugin 100%[===================>] 10.98M 8.32MB/s in 1.3s

2021-04-15 08:51:24 (8.32 MB/s) - ‘hplip-3.20.3-plugin.run.2’ saved [11514166/11514166]

$ sh hplip-3.20.3-plugin.run
Verifying archive integrity... All good.
Uncompressing HPLIP 3.20.3 Plugin Self Extracting Archive..............................................................
Error setting home directory: /etc/hp/hplip.conf not found. Is HPLIP installed?
$
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I looked in the directory /etc/hp and it was indeed empty!

See this question for more info:
https://answers.launchpad.net/hplip/+question/696517