Unable to update wordpress and plugins, what should I do?

Question

Access rights

Tell me please. There is a site on WP, installed it, uploaded plugins and content. At the beginning, everything was as it should, but at some point, when it was necessary to update WP itself and plugins, it began to be written during the update:

Warning: copy(/home/site/domains/sitename.ru/public_html/wp-admin/includes/update-core.php): failed to open stream: Permission denied in /home/site/domains/site.ru/public_html/wp-admin/includes/class-wp-filesystem-direct.php on line 231

Обновление не может быть установлено, потому что нам не удастся скопировать некоторые файлы. Обычно это связано с неверными правами на файлы.: wp-admin/includes/update-core.php

We set permissions to 777 for these files, but when you try to update again, a similar message appears with other files. You can’t put 777 on everything.
Googled, they write that this could be because all the files were originally uploaded under the root, and now WP is trying to update under a different user. But how is this possible if initially there was only one single user ..?
How can this be resolved?

Answer 1

Root server, change the owner of files and directories on the server

Answer 2

Another possible reason is due to the installed similarity of All In One WP Security plugins.
After disabling the All In One WP Security plugin, the update of other plugins reproduced successfully for me.