this page doesn't work anymore http://www.zenphoto.org/pages/nightly-builds.
there is no link to nightly builds...

The page works for me. Your link does not, however, because there is a period at the end.

http://www.zenphoto.org/pages/nightly-builds

There was a problem with the site for a little while today, so maybe that caused the problems.

the page still doesn't work for me this morning (the page exists, but it is partially loaded, and I can't see the latest nightly builds trunk and dev).

Yes, and there are other parts on the site that do not show either.
There are still some issues with the site.
The parts that do not show depend on a plugin that had to be disabled as otherwise the complete site would be down.
Our developers are busy fixing this.

The site should now work again.

The nightly and older versions download pages are up and working again.