WordPress installations suddenly don't have permissions

I had two fully functioning WordPress installations on one VPS. However, the auto-update to WordPress 5.0 failed and I had to manually upload it. After that it worked just fine, but now all of the sudden I can't do anything - update plugins (error while creating new directory), update WordPress, upload media files. This is happening on both WP sites.

In Filezilla all folders in /var/www appear to have the www-data user/group (?) assigned. And the owner has 755 permission.