Redmine

bundle exec rake assets:precompile RAILS_ENV=production
- not helped

According to the messages in the browser's debug console, the issue appears to be with the virtual host configuration in Apache2 (?):

GET
https://my_site_url/assets/jstoolbar/underline-7f1532c4.svg
[HTTP/1.1 403 Forbidden 1ms]

For some reason, after upgrading from 5.1.x or 6.0.7 to 6.1.0, the Redmine root, which should be https://my_site_url/redmine/ and consequently
https://my_site_url/redmine/assets/jstoolbar/underline-7f1532c4.svg, is being ignored for the /var/www/redmine/assets directory.

However, I have not changed the Apache2 virtual host configuration since the beginning, for all Redmine versions I've used since 4.x.x.

Currently digging in this direction.

Exactly. The issue was in the order of Apache directive processing. The <LocationMatch "^(?!/(redmine|dokuwiki))"> directive was blocking access BEFORE the request could reach Passenger.

Solution: Move the restrictive LocationMatch directive below the Redmine configuration blocks to prevent it from blocking access BEFORE the request reaches Passenger.

Issue is resolved, can be closed.