I mentioned everything I know in my several posts, what’s not clear to you ?
I can make a short resume here :
Initial problem was that Apache was not taking into account my
/etc/apache2/envvars file.
This is what I’ve put :
export APACHE_RUN_USER=my_user
export APACHE_RUN_GROUP=my_user
I was using the wrong command to restart apache, which was not easy to see because when I was restarting apache using sudo systemctl reload apache2 (which is the command given in Grav documentation), apache was reloading but maybe not to the point that it would take the envvars file into account. I don’t know anything about Apache so this is not really clear to me.
The right command so Apache will reload the envvars file in Debian is this one :
sudo /etc/init.d/apache2 restart
Then Grav could not work properly as the “session” file owner in /var/lib/php/sessions/ was www-data.
I’ve run this command to delete it (it will be recreated next time Apache will load Grav or php, I don’t really understand how all this works)
sudo sh -c "rm -rf /var/lib/php/sessions/*"
I also changed the ownership of all my website files, to be sure everything is right :
sudo chown -R user:user /var/www/my_website/*
That’s it.