Update via ftpsockets fail

Hello all,

Here is my problem while trying to update either the core or the plugins via the web interface it will fail with the following message:
Update Failed: Could not copy file. add-to-any/addtoany.admin.js

This example being with the plugin addtoany share buttons but it does the same with the core or other plugins.

Wordpress version is 4.9.5. I got access to both FTP and SSH but have to be able to update via the web ui only.
Here are a few relevant informations about the constants defined in wp-config.php and directories in the file system:

define('FS_METHOD', 'ftpsockets');
define('FTP_BASE', '/path/to/wordpress' );
define('FTP_CONTENT_DIR', '/path/to/wordpress/wp-content/' );
define('FTP_PLUGIN_DIR ', '/path/to/wordpress/wp-content/plugins/' );
define('FTP_USER', 'ftpuser' );
define('FTP_PASS', 'ftppwd' );
define('FTP_HOST', '127.0.0.1' );
define('WP_DEBUG_LOG', true);
define('WP_TEMP_DIR', '/path/to/custom/tmp');

Some values have been edited to protect the innocent, but all FTP credentials works fine, same as the TMP dir being writable by www-data and I’ve witnessed it storing the downloaded archive inside.
the wp-content/upgrade folder is writable by the FTP user.
the wp-content/plugins folder is writable both by the FTP user and the PHP user

The error message on top of my post is the most information I managed to squeeze out of wordpress despite the WP_DEBUG constant set to true and access to error logs, not knowing the complete path, from where, to where the file is tried to be copied by wordpress is a problem as well.

I’m available if more informations are needed, please advise.