Is there a big orange exclamation mark on the icon?
Is the .desktop file executable?

Dear David
No, there is no big orange exclamation mark on the icon.
.desktop file is executable

But I found this after running "kioclient5 exex myFile.desktop" on command line:

grep: /proc/sys/fs/binfmt_misc/WSLInterop: No such file or directory
WSL Interopability is disabled. Please enable it before using WSL.
grep: /proc/sys/fs/binfmt_misc/WSLInterop: No such file or directory
[error] WSL Interoperability is disabled. Please enable it before using WSL.
/usr/bin/wslview: line 216: /mnt/c/Windows/System32/reg.exe: No such file or directory
/usr/bin/wslview: line 308: [: -ge: unary operator expected
[error] This protocol is not supported before version 1903.

So we are starting the program and you have some other client related issue.

What does that mean?
This is as installed.
Only kioclient5 i have installed because on KUBUNTU 20 I had a similar issue which could be solved by installing kioclient5.

Let me rephrase.

If you create a desktop file on the desktop that opens dolphin does everything work correctly?

I tried to use dolphin. But the click from the desktop doesn't work either.
Dolphin from command line works but not by double click on desktop icon.
! File Associations have been adapted to dolphin !

if I open the .desktop file from the dolphin application itself (double click onto the file within the dolphin window) - that works.

please attach your .desktop file

one example:

[Desktop Entry]
Encoding=UTF-8
Icon=/home/wolf/icons/money_compass.png
Type=Link
URL[$e]=file://$HOME/wdata/privat/anlagen/Dividenden/Div.ods

When I create a file like that (and replace the URL with a file that exists) it opens fine

It looks like the app for opening that file starts and crashes/quits immediately

But this is not only the case when .desktop files should be executed, this is also the case when I just drop a file onto the desktop and choose "icon". After that, and I double click the icon, nothing happen either!

[Desktop Entry]
Exec=/usr/bin/soffice /home/wolf/wdata/temp/firefighting.odg
Icon=/home/wolf/icons/fire.png
Name=fire
Path=/usr/bin
StartupNotify=true
Terminal=false
Type=Application

That works fine, because the application is specified.
Without specification of the app it doesn't work

Dear David,
Better we close that issue.
Due to a lot of other "unexplainable" issues on my system I have erased it from the hard disc and installed it brand new. Now everything is working fine.
Mot likely during some kind of trials of my side, the system was contaminated with some rubbish which I cannot trace anymore.
So let's close it.
Thank you for your try to help me.

Best regards

Wolf