CSS and JS aggregation not working

Hi,

just made a fresh Backdrop test installation, v1.28.0. Environment: Debian 10, Apache 2.4.38, MariaDB 5.5.5-10.3.39-MariaDB-0+deb10u2, PHP 7.3.31-1~deb10u6. Status report at ./admin/reports/status looks good. Only vanilla Backdrop, no contrib modules or own code modifications.

Installer ran without CSS for unknown reasons (ugly but still works); when the installation was finished, I disabled "Aggregate and compress CSS files" and "Aggregate JavaScript files" at ./admin/config/development/performance. Now I'm getting formatted pages, including the admin bar on top of the site, and with icons in it.

Permissions for css and js subdirectories in ./files:

<blockquote>$ ls -lah files
drwxrwx--- 2 debian www-data 4.0K May 19 03:28 css
drwxrwx--- 2 debian www-data 4.0K May 19 03:30 js</blockquote>

'debian' is the server user, 'www-data' is the default Apache daemon user on Debian and derivates. Not sure what I am doing wrong.

Also I noticed that there are two broken images on the front page ("card3-organize.png" and "card2-card.png"). 

<blockquote>$ ls -lah files/field/image/
-rwxrwx--- 1 debian www-data 1.5K May 19 03:14 card1-layout.png
-rwxrwx--- 1 debian www-data 3.0K May 19 03:14 card2-card.png
-rwxrwx--- 1 debian www-data 1.2K May 19 03:14 card3-organize.png</blockquote>

Images files exist where they are supposed to be, and they can be loaded in the web browser with the full URL (e.g. http://{mytestsite}/files/styles/card/public/field/image/card3-organize.png). They are 'just' not loaded on the front page.

When I upload an image file, the upload works fine. Image shows up in the ./files/field/image directory, but not in the web page. It gets the following permissions: 

<blockquote>-rw-rw-r-- 1 www-data www-data  74K May 19 16:19 image.jpg</blockquote>

The page source points to this Imagecache location:

<blockquote>$ ls -lah files/styles/large/public/field/image/
-rw-rw-r-- 1 www-data www-data  38K May 19 16:23 image.jpg</blockquote>

Again, with the full URL, the image can be loaded from the Imagecache location.

Similar experience with the "Manage files" feature at ./admin/content/files. The images are listed, but can not be displayed when clicking on the file name. But they load fine with the full URL.

Did a "Flush all caches" a couple of times, this odd behaviour still persists.

Also tried this from the Backdrop installation root:

<blockquote>$ sudo chown -R :www-data files
$ sudo chmod g+ws files
$ sudo find . -type d -exec chmod g+ws {} \;
$ sudo find . -type f -exec chmod 664 {} \;</blockquote>

This set of commands I'm using for years to fix file permissions of Drupal sites when I had to restore a backup. In this case, they seem not to work, or Backdrop expects other permissions.

Any suggestions what I am missing?

Thanks!