Has anyone taken rather a trivial step of running systemsettings from the terminal and seeing whether any error messages show up ?
On Ubuntu 24.04.1 LTS | KWin version: 5.27.11 | Qt compile version: 5.15.13 the following is output:
kf.newstuff.core: Could not find category "121"
Each category inside store.kde.org has a corresponding category id.
In this case 121 maps to Global Themes (Plasma 5) - KDE Store which is Global Themes (Plasma 5)
The framework knewstuff makes use of configuration files inside /usr/share/knsrcfiles
Backup the following file and then edit /usr/share/knsrcfiles/lookandfeel.knsrc
Update
Categories=121
With
Categories=Global Themes (Plasma 5)
Then try to get new themes again.