When you are here: https://www.phpbb.com/customise/db/extensions-36

And then click on the left side on "Official extensions", which is done by a AJAX loading.

The breadcrumbs shows:
"Board index > Customisation Database > Extensions > Official Extensions"

The "board index" links to: https://www.phpbb.com/customise/community/index.php which gives a 404 error.

If you access https://www.phpbb.com/customise/db/extensions/official-extensions-86 directly, this issue it not showing itself.