OmekaS, php, can't write their error log file, seemingly with correct permissions

Question

I am currently installing and configuring an instance of OmekaS, a CMS for libraries (in short).

I am encountering an error when trying to write to log files.

They should be in /var/www/CMS/log/application.log; but this file stays empty no matter what.

When checking /var/log/apache2/error/log, i get this :

 [php7:notice] [pid 1447] [client 172.18.1.181:2898] [Omeka S] File logging disabled: not writeable., referer: https://193.48.29.1/admin/log

However, this is surprising, as when i do ls -la in this folder i get

drwxrwxr-x  2 www-data      www-data        4096 Mar 29 02:33 log

for the folder and

-rwxrwxrwx  1 www-data www-data   33 Mar 29 02:33 application.log

for the file. I have even tried setting it to 777.

I read the documentation and set the config files written : https://omeka.org/s/docs/user-manual/errorLogging/

And this is the bit of code from the CMS returning the error :

            if (!is_file($writers['stream']['options']['stream'])
            || !is_writeable($writers['stream']['options']['stream'])
        ) {
            error_log('[Omeka S] File logging disabled: not writeable.'); // @translate
            unset($writers['stream']);
            if (empty($writers)) {
                return (new Logger)->addWriter(new Noop);
            }
        }

I have tried creating a dummy php file that only create a file, and it works.

I have no idea what's wrong

Thanks