Replying to myself:

The bugrepport said is is fixed for 5.10.1

And it can be made to work with correct folder permissions :

$ ls -ld /etc/config/ drwxr-xr-x 2 root root 4096 avril 1 11:15 /etc/config/ $ sudo chgrp pi /etc/config/ $ sudo chmod g+w /etc/config/ $ ls -ld /etc/cleandrop/ drwxrwxr-x 2 root pi 4096 avril 1 11:15 /etc/config/

resolving simlink doesn't helps

// resolve symlink // QFileInfo info(_configuration_file); if (info.isSymLink()) _configuration_file = info.symLinkTarget();

So I removed this fix, and fixed the folder permission and it worked.