Hi,
I have the following problem on Kubuntu 20.04 LTS:

I can not start any program by double clicking on the .desktop file.
I can start it by clicking on it once => then hit enter on the keyboard.
I can start programs from taskbar.
In dolphin double mouse click works as I set it in the settings. => double click opens folders and starts runables.

I have not yet found anything similar to my issue on google.

Hi!

The default action in Kubuntu is to open .desktop-files with Kate as there are textfiles. If you want to change this, rightclick one, hit open with… → other application and write xdg-open and mark the 'Remember app assoc'-checkbox at the bottom. This will create the desktop-file ~/.local/share/applications/xdg-open.desktop which will then be started to open other desktop-files on click.

But why would you do this? It is much easier to open applications via KRunner or use LatteDock, the default panel… whatever.