I've been receiving this error for several years in all my Wordpress installations. The solution has been always the same: the classic snipet where you add a filter for "filesystem_method" with direct mode.
The fact is that for some reason, today I've done a new installation in a Ubuntu 18.04 server and this error has not arised. But when installing my WPML plugin something similar happened: https://wpml.org/forums/topic/cannot-write-into-the-plugins-folder-but-folder-is-chmod-775/
Exactly the same to this user, but as you may see, the WPML support don't know how to solve it. The solution? With the same classic filesystem_method snippet.
But at this point I think noone has stopped to think what it's happening. And personally I don't even know what's happening.
I have Dedicated servers and VPS with all my WP installations. And the problem keeps happening. ALL have Apache2 (so maybe in Nginx this is not a problem).
Most of my WP installations have been running with FULL 777 permisions over the WHOLE wordpress folder (recursively chmod 777 -R the_whole_wordpress_folder) And still I've been reading and receiving as an answer: "You have to set chown to the uploads folder to solve this issue". If you understand the 777 set, ALL users including guest, have write-read and execution permissions therefore it doesnt matter if the apache2 is the owner or in the same group owner of the folder.
This said, is still awkward for me, that the only way to make this work, is to adjust the Direct method
BTW: The define('FS_METHOD', 'direct'); also works exactly the same, as changing the filesystem method filter
So here it's my question: Why FS_METHOD has to be set to Direct to perform this actions even under a full 777'ish wordpress installation? What's going on here?