Hello Stefan, here we are.

Removing "ppa:x2go/stable":

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root@noc /var/log/apt # sudo apt-add-repository -r ppa:x2go/stable
 Quick howto to turn your machine into an X2Go server:

sudo apt-add-repository ppa:x2go/stable
sudo apt-get update
sudo apt-get install x2goserver x2goserver-xsession
sudo apt-get install x2gomatebindings  # if you use MATE/mubuntu
sudo apt-get install x2golxdebindings  # if you use LXDE/lubuntu

Quick howto to install the x2goclient:

sudo apt-add-repository ppa:x2go/stable
sudo apt-get update
sudo apt-get install x2goclient

Enjoy!
 More info: https://launchpad.net/~x2go/+archive/ubuntu/stable
Press [ENTER] to continue or Ctrl-c to cancel removing it.
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Removing "ppa:x2go/saimaa" also:

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root@noc /var/log/apt # sudo apt-add-repository -r ppa:x2go/saimaa
 Bundle release of X2Go named ,,Saimaa'' (based on X2Go Server / Client 4 / nx-libs 3.5.0)
 More info: https://launchpad.net/~x2go/+archive/ubuntu/saimaa
Press [ENTER] to continue or Ctrl-c to cancel removing it.

root@noc /var/log/apt # sudo apt update
Hit:1 http://mirror.hetzner.de/ubuntu/packages focal InRelease
Hit:2 http://mirror.hetzner.de/ubuntu/packages focal-updates InRelease                                                                                                                                                
Hit:3 http://mirror.hetzner.de/ubuntu/packages focal-backports InRelease                                                                                                                                              
Hit:4 http://mirror.hetzner.de/ubuntu/packages focal-security InRelease                                                                                                                                               
Hit:5 http://de.archive.ubuntu.com/ubuntu focal InRelease                                                                                                                                                             
Hit:6 http://ppa.launchpad.net/kubuntu-ppa/backports/ubuntu focal InRelease                                                                                                                                           
Hit:7 http://download.virtualbox.org/virtualbox/debian hirsute InRelease                                                                                                                    
Hit:8 http://de.archive.ubuntu.com/ubuntu focal-updates InRelease                                                                                                        
Ign:9 http://linux.dropbox.com/ubuntu disco InRelease                                                                  
Hit:10 http://linux.dropbox.com/ubuntu disco Release                                             
Get:11 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB]                       
Hit:12 http://de.archive.ubuntu.com/ubuntu focal-backports InRelease                                        
Get:13 https://esm.ubuntu.com/infra/ubuntu focal-infra-security InRelease [7,453 B]                         
Get:14 https://esm.ubuntu.com/infra/ubuntu focal-infra-updates InRelease [7,452 B]
Get:16 http://security.ubuntu.com/ubuntu focal-security/main i386 Packages [602 kB]
Get:17 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages [2,209 kB]
Fetched 2,940 kB in 6s (496 kB/s)                            
Reading package lists... Done
Building dependency tree       
Reading state information... Done
25 packages can be upgraded. Run 'apt list --upgradable' to see them.
N: Skipping acquire of configured file 'non-free/binary-i386/Packages' as repository 'http://download.virtualbox.org/virtualbox/debian hirsute InRelease' doesn't support architecture 'i386'
N: Skipping acquire of configured file 'contrib/binary-i386/Packages' as repository 'http://download.virtualbox.org/virtualbox/debian hirsute InRelease' doesn't support architecture 'i386'
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Now the "sudo apt upgrade -s" part:

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root@noc /var/log/apt # sudo apt upgrade -s
Reading package lists... Done
Building dependency tree       
Reading state information... Done
You might want to run 'apt --fix-broken install' to correct these.
The following packages have unmet dependencies:
 libx2go-log-perl : Depends: x2goserver-common (>= 4.1.0.4-0~1975~ubuntu20.04.1) but 4.1.0.3-5 is installed
 libx2go-server-db-perl : Depends: x2goserver-common (>= 4.1.0.4-0~1975~ubuntu20.04.1) but 4.1.0.3-5 is installed
 libx2go-server-perl : Depends: libx2go-log-perl (< 4.1.0.3-5.1~) but 4.1.0.4-0~1975~ubuntu20.04.1 is installed
                       Depends: libx2go-server-db-perl (< 4.1.0.3-5.1~) but 4.1.0.4-0~1975~ubuntu20.04.1 is installed
E: Unmet dependencies. Try 'apt --fix-broken install' with no packages (or specify a solution).
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As you can see there is not the expected output. So I went ahead with "apt list --upgradable":

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root@noc /var/log/apt # apt list --upgradable
Listing... Done
apport-kde/focal-updates,focal-updates,focal-updates,focal-updates 2.20.11-0ubuntu27.27 all [upgradable from: 2.20.11-0ubuntu27.26]
apport/focal-updates,focal-updates,focal-updates,focal-updates 2.20.11-0ubuntu27.27 all [upgradable from: 2.20.11-0ubuntu27.26]
cups-bsd/focal-security 2.3.1-9ubuntu1.3 amd64 [upgradable from: 2.3.1-9ubuntu1.2]
cups-client/focal-security 2.3.1-9ubuntu1.3 amd64 [upgradable from: 2.3.1-9ubuntu1.2]
cups-common/focal-security,focal-security 2.3.1-9ubuntu1.3 all [upgradable from: 2.3.1-9ubuntu1.2]
cups-core-drivers/focal-security 2.3.1-9ubuntu1.3 amd64 [upgradable from: 2.3.1-9ubuntu1.2]
cups-daemon/focal-security 2.3.1-9ubuntu1.3 amd64 [upgradable from: 2.3.1-9ubuntu1.2]
cups-ipp-utils/focal-security 2.3.1-9ubuntu1.3 amd64 [upgradable from: 2.3.1-9ubuntu1.2]
cups-ppdc/focal-security 2.3.1-9ubuntu1.3 amd64 [upgradable from: 2.3.1-9ubuntu1.2]
cups-server-common/focal-security,focal-security 2.3.1-9ubuntu1.3 all [upgradable from: 2.3.1-9ubuntu1.2]
cups/focal-security 2.3.1-9ubuntu1.3 amd64 [upgradable from: 2.3.1-9ubuntu1.2]
iptables/focal-updates,focal-updates 1.8.4-3ubuntu2.1 amd64 [upgradable from: 1.8.4-3ubuntu2]
libcups2/focal-security 2.3.1-9ubuntu1.3 amd64 [upgradable from: 2.3.1-9ubuntu1.2]
libcupsimage2/focal-security 2.3.1-9ubuntu1.3 amd64 [upgradable from: 2.3.1-9ubuntu1.2]
libip4tc2/focal-updates,focal-updates 1.8.4-3ubuntu2.1 amd64 [upgradable from: 1.8.4-3ubuntu2]
libip6tc2/focal-updates,focal-updates 1.8.4-3ubuntu2.1 amd64 [upgradable from: 1.8.4-3ubuntu2]
libssl1.1/focal-updates,focal-security,focal-updates,focal-security 1.1.1f-1ubuntu2.19 amd64 [upgradable from: 1.1.1f-1ubuntu2.18]
libxtables12/focal-updates,focal-updates 1.8.4-3ubuntu2.1 amd64 [upgradable from: 1.8.4-3ubuntu2]
linux-image-generic/focal-updates,focal-updates 5.4.0.150.148 amd64 [upgradable from: 5.4.0.149.147]
linux-libc-dev/focal-updates,focal-updates 5.4.0-150.167 amd64 [upgradable from: 5.4.0-149.166]
openssl/focal-updates,focal-security,focal-updates,focal-security 1.1.1f-1ubuntu2.19 amd64 [upgradable from: 1.1.1f-1ubuntu2.18]
python3-apport/focal-updates,focal-updates,focal-updates,focal-updates 2.20.11-0ubuntu27.27 all [upgradable from: 2.20.11-0ubuntu27.26]
python3-problem-report/focal-updates,focal-updates,focal-updates,focal-updates 2.20.11-0ubuntu27.27 all [upgradable from: 2.20.11-0ubuntu27.26]
snapd/focal-updates,focal-security,focal-updates,focal-security 2.58+20.04.1 amd64 [upgradable from: 2.58+20.04]
tzdata/focal-updates,focal-updates,focal-updates,focal-updates 2023c-0ubuntu0.20.04.2 all [upgradable from: 2023c-0ubuntu0.20.04.1]
root@noc /var/log/apt #
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As far as I can see we should get rid of these orphan packages:

Is that correct? Thanks again, and best regards, e.-