I found the solution 
In the post linked from Profile - JackGruber - Joplin Forum Problem using external editor - #11 by james-carroll there is a link to another post: Launch external editor (Linux, Ubuntu/PopOS, flatpak) - #3 by fnl
One amendment for completeness, if others have a similar issue and if you want to spawn non-flatpak apps from a flatpak-installed Joplin, such as Typora (which in turn isn't using flatpak), you can use this configuration instead:
Path:
/ bin / flatpak-spawn
Arguments:
--host / usr / bin / typora
I did that and it works!
!Tada!
Many thanks for the help