Update failing due to file permissions?

Totally new to WordPress and Asgaros:

Site is self-hosted on local server for development and testing.

I just update WordPress and all my plugins and themes except Asgaros fails. The error message claims file permissions, but everything under /var/www/ is owned by www-data:www-data.

Here is the complete error message:

An error occurred while updating Asgaros Forum: The update cannot be installed because we will be unable to copy some files. This is usually due to inconsistent file permissions. libs, libs/fontawesome, libs/fontawesome/webfonts, libs/fontawesome/webfonts/fa-regular-400.svg, libs/fontawesome/webfonts/fa-solid-900.eot, libs/fontawesome/webfonts/fa-regular-400.eot, libs/fontawesome/webfonts/fa-solid-900.svg, libs/fontawesome/webfonts/fa-brands-400.ttf, libs/fontawesome/webfonts/fa-solid-900.ttf, libs/fontawesome/webfonts/fa-brands-400.woff, libs/fontawesome/webfonts/fa-regular-400.woff, libs/fontawesome/webfonts/fa-brands-400.eot, libs/fontawesome/webfonts/fa-regular-400.woff2, libs/fontawesome/webfonts/fa-brands-400.woff2, libs/fontawesome/webfonts/fa-solid-900.woff, libs/fontawesome/webfonts/fa-solid-900.woff2, libs/fontawesome/webfonts/fa-brands-400.svg, libs/fontawesome/webfonts/fa-regular-400.ttf, libs/fontawesome/css, libs/fontawesome/css/all.min.css, libs/fontawesome/css/v4-shims.min.css, libs/fontawesome/LICENSE.txt, uninstall.php, skin, skin/widgets.css, skin/style.css, skin/rtl.css, skin/editor.css, widgets, widgets/widget-recent-topics.php, widgets/widget-search.php, widgets/widget-recent-posts.php, includes, includes/forum-usergroups.php, includes/forum-appearance.php, includes/forum-unread.php, includes/forum-shortcodes.php, includes/forum-statistics.php, includes/forum-editor.php, includes/forum-feed.php, includes/forum-rewrite.php, includes/forum-widgets.php, includes/forum-pagination.php, includes/forum-polls.php, includes/forum-permissions.php, includes/forum-uploads.php, includes/forum-reports.php, includes/forum-breadcrumbs.php, includes/forum.php, includes/forum-search.php, includes/forum-activity.php, includes/forum-approval.php, includes/forum-content.php, includes/forum-profile.php, includes/views, includes/views/topic.php, includes/views/post-element.php, includes/views/forum-element.php, includes/views/overview.php, includes/views/forum.php, includes/forum-notifications.php, includes/forum-reactions.php, includes/forum-memberslist.php, includes/forum-spoilers.php, includes/forum-database.php, includes/forum-online.php, includes/forum-mentioning.php, includes/forum-compatibility.php, includes/forum-ads.php, readme.txt, languages, languages/asgaros-forum.pot, admin, admin/css, admin/css/admin.css, admin/tables, admin/tables/admin-structure-table.php, admin/tables/admin-ads-table.php, admin/tables/admin-usergroups-table.php, admin/images, admin/images/logo.png, admin/admin.php, admin/views, admin/views/ads.php, admin/views/appearance.php, admin/views/usergroups.php, admin/views/options.php, admin/views/structure.php, admin/js, admin/js/admin.js, js, js/script-spoilers.js, js/script.js, asgaros-forum.php