Ubuntu Software has changed

https://askubuntu.com/questions/78561/software-center-window-is-blank

that doesn't relate to my question.

For diagnostic purposes: What is the output of the commands

uname -a
lsb_release -crid
apt-cache policy ubuntu-software

Hello Manfred H., here is the requested output:

thomas@thomas-HP-ProDesk-400-G1-MT:~$ uname -a
Linux thomas-HP-ProDesk-400-G1-MT 4.15.0-29-generic #31~16.04.1-Ubuntu SMP Wed Jul 18 08:54:04 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
thomas@thomas-HP-ProDesk-400-G1-MT:~$ lsb_release -crid
Distributor ID: Ubuntu
Description: Ubuntu 16.04.5 LTS
Release: 16.04
Codename: xenial
thomas@thomas-HP-ProDesk-400-G1-MT:~$ apt-cache policy ubuntu-software
ubuntu-software:
  Installiert: 3.20.5-0ubuntu0.16.04.11
  Installationskandidat: 3.20.5-0ubuntu0.16.04.11
  Versionstabelle:
 *** 3.20.5-0ubuntu0.16.04.11 500
        500 http://de.archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     3.20.1+git20160420.1.ca63436.ubuntu-xenial-0ubuntu2 500
        500 http://de.archive.ubuntu.com/ubuntu xenial/main amd64 Packages
W: Target Packages (partner/binary-amd64/Packages) is configured multiple times in /etc/apt/sources.list:43 and /etc/apt/sources.list.d/xenial-partner.list:4
W: Target Packages (partner/binary-i386/Packages) is configured multiple times in /etc/apt/sources.list:43 and /etc/apt/sources.list.d/xenial-partner.list:4
W: Target Packages (partner/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:43 and /etc/apt/sources.list.d/xenial-partner.list:4
W: Target Translations (partner/i18n/Translation-de_DE) is configured multiple times in /etc/apt/sources.list:43 and /etc/apt/sources.list.d/xenial-partner.list:4
W: Target Translations (partner/i18n/Translation-de) is configured multiple times in /etc/apt/sources.list:43 and /etc/apt/sources.list.d/xenial-partner.list:4
W: Target Translations (partner/i18n/Translation-en) is configured multiple times in /etc/apt/sources.list:43 and /etc/apt/sources.list.d/xenial-partner.list:4
W: Target DEP-11 (partner/dep11/Components-amd64.yml) is configured multiple times in /etc/apt/sources.list:43 and /etc/apt/sources.list.d/xenial-partner.list:4
W: Target DEP-11-icons (partner/dep11/icons-64x64.tar) is configured multiple times in /etc/apt/sources.list:43 and /etc/apt/sources.list.d/xenial-partner.list:4
thomas@thomas-HP-ProDesk-400-G1-MT:~$

and a Photo of the ubuntu-software:
/home/thomas/Bilder/Bildschirmfoto vom 2018-07-28 18-07-08.png

sudo rm /etc/apt/sources.list.d/xenial-partner.list
sudo apt-get update

Your image link only works on your computer. We don't have and do not want access. Use a service like imageshack or Dropbox public to make a URL of the image

Hello actionparsnip,
sudo rm /etc/apt/sources.list.d/xenial-partner.list
sudo apt-get update

...worked fine, but changed nothing on the "Ubuntu Software".
In the "Ubuntu Software" are only shown apps from the snap-store and no apt packages.
This began in january or february this year after a system-update for the Ubuntu Software to v3.20.5.

an photo of the ubuntu-software is at askubuntu.com under:
"apt packages are missing from Ubuntu Software" 2 days ago (toffy51)

You can try it, by installing ubuntu xenial 16.04.2(or .3) in e.g. VirtualBox without updates during installation.
Look at the Ubuntu Software working fine, it shows apt packages and apps from the snap-store. After the first
system updates (there also an update for the Ubuntu Software) the Ubuntu Software shows
only apps from the snap-store and no more apt-packages.

I think something is wrong with the Update to Ubuntu Software v3.20.5.

What is now the output of the command

sudo apt-get update

(Maybe your problem is caused by Bug #1780520 or Bug #1780442 ).

Hello Manfred H., here is the requested output:

From my home computer with Ubuntu Xenial 64bit (16.04lts in dualboot with Win7):

thomas@thomas-HP-ProDesk-400-G1-MT:~$ sudo rm /etc/apt/sources.list.d/xenial-partner.list
[sudo] Passwort für thomas:
thomas@thomas-HP-ProDesk-400-G1-MT:~$ sudo apt-get update
OK:1 http://archive.canonical.com/ubuntu xenial InRelease
OK:2 http://de.archive.ubuntu.com/ubuntu xenial InRelease
Holen:3 http://security.ubuntu.com/ubuntu xenial-security InRelease [107 kB]
OK:4 http://ppa.launchpad.net/appgrid/stable/ubuntu xenial InRelease
Holen:5 http://de.archive.ubuntu.com/ubuntu xenial-updates InRelease [109 kB]
OK:6 http://ppa.launchpad.net/atareao/atareao/ubuntu xenial InRelease
OK:7 http://ppa.launchpad.net/audio-recorder/ppa/ubuntu xenial InRelease
Holen:8 http://de.archive.ubuntu.com/ubuntu xenial-backports InRelease [107 kB]
OK:9 http://ppa.launchpad.net/freecad-maintainers/freecad-stable/ubuntu xenial InRelease
Holen:10 http://security.ubuntu.com/ubuntu xenial-security/main amd64 Packages [529 kB]
Holen:11 http://de.archive.ubuntu.com/ubuntu xenial-updates/main amd64 DEP-11 Metadata [318 kB]
OK:12 http://ppa.launchpad.net/gerardpuig/ppa/ubuntu xenial InRelease
OK:13 http://ppa.launchpad.net/libreoffice/libreoffice-6-0/ubuntu xenial InRelease
OK:14 http://ppa.launchpad.net/noobslab/themes/ubuntu xenial InRelease
OK:15 http://ppa.launchpad.net/thomas-schiex/blender/ubuntu xenial InRelease
OK:16 http://ppa.launchpad.net/webupd8team/indicator-kdeconnect/ubuntu xenial InRelease
Holen:17 http://de.archive.ubuntu.com/ubuntu xenial-updates/main DEP-11 64x64 Icons [232 kB]
Holen:18 http://security.ubuntu.com/ubuntu xenial-security/main i386 Packages [466 kB]
Holen:19 http://de.archive.ubuntu.com/ubuntu xenial-updates/universe amd64 Packages [675 kB]
Holen:20 http://security.ubuntu.com/ubuntu xenial-security/main amd64 DEP-11 Metadata [67,7 kB]
Holen:21 http://security.ubuntu.com/ubuntu xenial-security/main DEP-11 64x64 Icons [68,0 kB]
Holen:22 http://security.ubuntu.com/ubuntu xenial-security/universe amd64 Packages [361 kB]
Holen:23 http://security.ubuntu.com/ubuntu xenial-security/universe i386 Packages [308 kB]
Holen:24 http://de.archive.ubuntu.com/ubuntu xenial-updates/universe i386 Packages [616 kB]
Holen:25 http://security.ubuntu.com/ubuntu xenial-security/universe Translation-en [136 kB]
Holen:26 http://security.ubuntu.com/ubuntu xenial-security/universe amd64 DEP-11 Metadata [107 kB]
Holen:27 http://security.ubuntu.com/ubuntu xenial-security/universe DEP-11 64x64 Icons [142 kB]
Holen:28 http://de.archive.ubuntu.com/ubuntu xenial-updates/universe Translation-en [272 kB]
Holen:29 http://de.archive.ubuntu.com/ubuntu xenial-updates/universe amd64 DEP-11 Metadata [247 kB]
Holen:30 http://de.archive.ubuntu.com/ubuntu xenial-updates/universe DEP-11 64x64 Icons [333 kB]
Holen:31 http://de.archive.ubuntu.com/ubuntu xenial-updates/multiverse amd64 DEP-11 Metadata [5.964 B]
Holen:32 http://de.archive.ubuntu.com/ubuntu xenial-updates/multiverse DEP-11 64x64 Icons [14,3 kB]
Holen:33 http://de.archive.ubuntu.com/ubuntu xenial-backports/main amd64 DEP-11 Metadata [3.328 B]
Holen:34 http://de.archive.ubuntu.com/ubuntu xenial-backports/universe amd64 DEP-11 Metadata [5.100 B]
Es wurden 5.230 kB in 7 s geholt (699 kB/s).
AppStream cache update completed, but some metadata was ignored due to errors.
Paketlisten werden gelesen... Fertig
thomas@thomas-HP-ProDesk-400-G1-MT:~$

and from a recently installation in VirtualBox (also Ubnutu Xenial 64bit), where the same problem with Ubuntu Software exists:

thomas@thomas-VirtualBox:~$ sudo apt-get update
[sudo] Passwort für thomas:
Ign:1 http://dl.google.com/linux/earth/deb stable InRelease
OK:2 http://archive.canonical.com/ubuntu xenial InRelease
OK:3 http://ppa.launchpad.net/libreoffice/ppa/ubuntu xenial InRelease
Holen:4 http://security.ubuntu.com/ubuntu xenial-security InRelease [107 kB]
OK:5 http://dl.google.com/linux/earth/deb stable Release
Holen:7 http://security.ubuntu.com/ubuntu xenial-security/main amd64 DEP-11 Metadata [67,7 kB]
Holen:8 http://security.ubuntu.com/ubuntu xenial-security/main DEP-11 64x64 Icons [68,0 kB]
Holen:9 http://security.ubuntu.com/ubuntu xenial-security/universe amd64 DEP-11 Metadata [107 kB]
Holen:10 http://security.ubuntu.com/ubuntu xenial-security/universe DEP-11 64x64 Icons [142 kB]
OK:11 http://de.archive.ubuntu.com/ubuntu xenial InRelease
Holen:12 http://de.archive.ubuntu.com/ubuntu xenial-updates InRelease [109 kB]
Holen:13 http://de.archive.ubuntu.com/ubuntu xenial-backports InRelease [107 kB]
Holen:14 http://de.archive.ubuntu.com/ubuntu xenial-updates/main amd64 DEP-11 Metadata [318 kB]
Holen:15 http://de.archive.ubuntu.com/ubuntu xenial-updates/main DEP-11 64x64 Icons [228 kB]
Holen:16 http://de.archive.ubuntu.com/ubuntu xenial-updates/universe amd64 DEP-11 Metadata [247 kB]
Holen:17 http://de.archive.ubuntu.com/ubuntu xenial-updates/universe DEP-11 64x64 Icons [333 kB]
Holen:18 http://de.archive.ubuntu.com/ubuntu xenial-updates/multiverse amd64 DEP-11 Metadata [5.960 B]
Holen:19 http://de.archive.ubuntu.com/ubuntu xenial-backports/main amd64 DEP-11 Metadata [3.328 B]
Holen:20 http://de.archive.ubuntu.com/ubuntu xenial-backports/universe amd64 DEP-11 Metadata [5.100 B]
Es wurden 1.848 kB in 6 s geholt (291 kB/s).
AppStream cache update completed, but some metadata was ignored due to errors.
Paketlisten werden gelesen... Fertig
N: Skipping acquire of configured file 'main/binary-i386/Packages' as repository 'http://dl.google.com/linux/earth/deb stable InRelease' doesn't support architecture 'i386'
thomas@thomas-VirtualBox:~$

The warning
N: Skipping acquire of configured file 'main/binary-i386/Packages' as repository 'http://dl.google.com/linux/earth/deb stable InRelease' doesn't support architecture 'i386'
can be cured by the following:

I assume that you have a file /etc/apt/sources.list.d/google-earth.list that contains (among others) the line

deb http://dl.google.com/linux/earth/deb/ stable main

You have to edit that file with the command

sudo gedit /etc/apt/sources.list.d/google-earth.list

and add an architecture-specific information , changing that line into

deb [arch=amd64] http://dl.google.com/linux/earth/deb/ stable main

Hello Manfred H.,
the file at /etc/apt/sources.list.d/google-earth.list was empty at all.
I have moved the file to the trash and then typed sudo apt-get update again.
Here is the new output:

thomas@thomas-HP-ProDesk-400-G1-MT:~$ sudo apt-get update
[sudo] Passwort für thomas:
OK:1 http://archive.canonical.com/ubuntu xenial InRelease
OK:2 http://de.archive.ubuntu.com/ubuntu xenial InRelease
Holen:3 http://security.ubuntu.com/ubuntu xenial-security InRelease [107 kB]
OK:4 http://de.archive.ubuntu.com/ubuntu xenial-updates InRelease
OK:5 http://de.archive.ubuntu.com/ubuntu xenial-backports InRelease
OK:6 http://ppa.launchpad.net/appgrid/stable/ubuntu xenial InRelease
OK:7 http://ppa.launchpad.net/atareao/atareao/ubuntu xenial InRelease
OK:8 http://ppa.launchpad.net/audio-recorder/ppa/ubuntu xenial InRelease
OK:9 http://ppa.launchpad.net/freecad-maintainers/freecad-stable/ubuntu xenial InRelease
OK:10 http://ppa.launchpad.net/gerardpuig/ppa/ubuntu xenial InRelease
OK:11 http://ppa.launchpad.net/libreoffice/libreoffice-6-0/ubuntu xenial InRelease
OK:12 http://ppa.launchpad.net/noobslab/themes/ubuntu xenial InRelease
OK:13 http://ppa.launchpad.net/thomas-schiex/blender/ubuntu xenial InRelease
OK:14 http://ppa.launchpad.net/ubuntu-desktop/gnome-software/ubuntu xenial InRelease
OK:15 http://ppa.launchpad.net/webupd8team/indicator-kdeconnect/ubuntu xenial InRelease
Es wurden 107 kB in 2 s geholt (38,2 kB/s).
Paketlisten werden gelesen... Fertig
thomas@thomas-HP-ProDesk-400-G1-MT:~$

And the Ubuntu Software will remain unchanged; just the one recommendation line of apps, probably from the snap-store
and no apt packages, like the photo at askubuntu.com (see above).

I will wait for the next Update for the Ubuntu Software package and hope that it will than show all
apt-packages and the installed apt-apps again.

The correct bug reference probably is Bug #1782534

Yes, the correct bug reference is Bug #1782534