Laurent, I’ll try to do this in stages. I have just isolated one machine and re-installed Joplin from scratch. In order to do that, I first deleted the the ~/.config/joplin-desktop directory and everything inside. Then a ran Joplin and it recreated the initial files. After that, I deleted the notebook “Welcome”.
Well, three files are still present in the directory ~/.config/joplin-desktop/resources. Those files are 2ec84055d2ea40c991643d5aeb62e6c1.png, 934f4e7f5241452a8e55cf023178e39b.png and d92de2ad3ba24a9c9645aa2f0e35156f.png.
There contents are as bellow.