Thank you for your great support.
The error message is correct. If I enter the URL
https://<domain>/wp-content/uploads/bp-attachments/190/test.png
into a logged-out web browser the document is seen.
I am using the Bitnami multisite distribution. This uses Apache and in httpd-app.conf I find
<Directory "/opt/bitnami/apps/wordpress/htdocs">
Options +MultiViews +FollowSymLinks
AllowOverride None
<IfVersion < 2.3 >
Order allow,deny
Allow from all
</IfVersion>
<IfVersion >= 2.3>
Require all granted
</IfVersion>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
# uploaded files
RewriteRule ^files/(.+) wp-includes/ms-files.php?file=$1 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . index.php [L]
<IfDefine USE_PHP_FPM>
RewriteEngine On
RewriteOptions Inherit
RewriteRule ^(.*\.php(/.*)?)$ fcgi://uds=%2fopt%2fbitnami%2fphp%2fvar%2frun%2fwordpress.sock/%{REQUEST_FILENAME} [P,L]
</IfDefine>
</Directory>
I understand that this is the wrong setting for AllowOverride.
Please tell me the correct setting for AllowOverride because this is not obvious from the Apache documentation.
Many thanks,
James