Folder.gif icon missing from derby documentation website

When I go to:

http://db.apache.org/derby/docs/10.9/

my browser displays "broken icon" symbols for the "icons/folder.gif" icon that's displayed next to each icon.

I think this is a new problem; last time I visited this URL (a few weeks ago?) I don't recall seeing this problem.

It appears to affect all the doc version (10.8, 10.7, etc.)

Note also that if I provide the index.html on the URL, as in

http://db.apache.org/derby/docs/10.9/ref/index.html

then the docs themselves are displayed correctly.

Perhaps this is part of the basic web server configuration, and not part of the Derby documentation proper, and indicates that something in the web server itself has recently been re-configured?