After updating to Ubuntu 22.04 beta, I had to realize that the desktop client no longer works via the Ubuntu ppa. The following dependencies could not be fulfilled:
Die folgenden Pakete haben unerfüllte Abhängigkeiten:
nextcloud-desktop : Hängt ab von: qtbase-abi-5-15-2 ist aber nicht installierbar
Empfiehlt: nextcloud-desktop-doc soll aber nicht installiert werden
E: Probleme können nicht korrigiert werden, Sie haben zurückgehaltene defekte Pakete.
No real idea. But perhaps you can uninstall all from the ppa source and install nextcloud-desktop (version 3.4.2) from Ubuntu 22.04 LTS.
Ubuntu – Details of package nextcloud-desktop in jammy
Also you can change to Debian Testing (Bockworm) Desktop. There you get acutally also Nextcloud Client 3.4.2 (https://packages.debian.org/bookworm/nextcloud-desktop).
Also you can:
Download here: https://nextcloud.com/de/install/#install-clients
and read (sorry not tested)
https://wiki.ubuntuusers.de/AppImage
Thank you, the nextcloud-desktop (version 3.4.2) from Ubuntu 22.04 LTS works for me. Uninstall the ppa source and no problems anymore. 
Ubuntu 22.04 LTS is a really new release of Ubuntu and all packages are almost new.
You actually get Nextcloud Desktop 3.4.2. The newest version is Nextcloud Desktop 3.4.4.
If you would use the LTS-release-path the Nextcloud Desktop package will be quite outdated in the next two years.
Ubuntu 20.04 LTS (two years old) is shipped with Nextcloud Desktop 2.6.2.
with LTS it makes sense to use ppa.
Ubuntu 21.10 is shipped with Nextcloud Desktop 3.2.3.
with no LTS it makes no really sense to use ppa.
Unless you already find this version too old.
Do you use only LTS Ubuntu versions or also the between releases?
I am in a similar situation after upgrading Ubuntu from 20.04 to 22.04. I have uninstalled and completely re-installed the Desktop client:
apt list nextcloud | grep installed
libnextcloudsync0/jammy,now 3.4.2-1ubuntu1 amd64 [installed,automatic]
nextcloud-desktop-common/jammy,jammy,now 3.4.2-1ubuntu1 all [installed,automatic]
nextcloud-desktop-doc/jammy,jammy,now 3.4.2-1ubuntu1 all [installed,automatic]
nextcloud-desktop-l10n/jammy,jammy,now 3.4.2-1ubuntu1 all [installed,automatic]
nextcloud-desktop/jammy,now 3.4.2-1ubuntu1 amd64 [installed]
but any try to connect to the server ends with a TLS issue. I even removed the account and attempted a new account set up at the Desktop client end, it will error with “TLS initialization failed”.
I do not understand the underlying dependencies in the stack but suspect it has something to do with QT5 not being fully installed/available on Ubuntu 22.04 after the upgrade (?)
Maybe you have got a problem with TLS on your new installation. Maybe there is only a problem with your Nextcloud and the used tls certificate.
You can create a 60 minutes test account at https://try.nextcloud.com . You get a 16 characters long account name with the passwort demo on the server https://demo1.nextcloud.com or https://demo2.nextcloud.com .
Please test with this account. Is there also a TLS problem? If yes it is an Ubuntu installation problem. If not it is a Nextcloud server problem.
Thanks for your suggestions and help.
To create a test account was easy and straight forward and works well.
Maybe I should have added earlier that access to our own Nextcloud server instance via web browser (https) is still working fine from my upgraded Ubuntu installation and also all of my colleagues with their (Windows or older Ubuntu) Desktop client installations are working fine so I cannot see a problem with the server side.
I think it is only a problem with my Ubuntu installation at a lower level, not affecting web browser access.
Nextcloud Client and Browser both uses TLS. Perhaps a CA problem. Are all needed certificates in Ubuntu (not browser but system) installed? Can you post the Nextcloud server CA structure?
Maybe a problem here: Ubuntu – Details of package ca-certificates in jammy
Maybe you can test with openssl command line client to your Nextcloud server to find an error. Read this.
Thanks for your continuing help.
I can get access from my preferred web browser (Firefox) to the Nextcloud demo account, no problem at all. When I try the Nextcloud client from my upgraded Ubuntu installation the local Nextcloud logfiles show the below (hope that is helpful; Oh, I am only allowed 4 links as a new user I am told)).
BTW, I tested the Nextcloud client on a fresh Ubuntu 22.04 installation and that connects nicely to our Nextcloud server.
…
2022-05-01 19:09:33:656 [ debug nextcloud.sync.cookiejar ./src/libsync/cookiejar.cpp:90 ] [ OCC::CookieJar::cookiesForUrl ]: QUrl(“https://demo1.nextcloud.com/status.php”) requests: ()
2022-05-01 19:09:33:657 [ info nextcloud.sync.networkjob ./src/libsync/abstractnetworkjob.cpp:361 ]: OCC::CheckServerJob created for “https://demo1.nextcloud.com/” + “status.php” “OCC::OwncloudSetupWizard”
2022-05-01 19:09:33:953 [ warning qt.network.ssl unknown:0 ]: QSslSocket::connectToHostEncrypted: TLS initialization failed
2022-05-01 19:09:33:953 [ warning qt.network.ssl unknown:0 ]: QSslSocket::connectToHostEncrypted: TLS initialization failed
2022-05-01 19:09:33:953 [ warning nextcloud.sync.networkjob ./src/libsync/abstractnetworkjob.cpp:224 ]: QNetworkReply::UnknownNetworkError “TLS initialization failed” QVariant(Invalid)
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