NextGEN Download Gallery v1.5.2 has just been released. It fixes a compatibility problem with NextGEN Gallery 2.1.7 and later.
NextGEN Gallery 2.1.7 brought a change to transient handling that affected the Download All button on the Download gallery. I’ve had to change how I handle the “gallery ID” for that button on NextGEN Gallery.
Quite likely, how I was handling it previously could have failed intermittently on some websites, especially websites with page caching. I used to rely on the ID of the transient cache for the gallery, which can become invalid after a period, or even right away in some circumstances.
I’m now encoding the terms used to build the gallery into a virtual ID; terms like which gallery ID or IDs, which image tag(s), and any excluded images. This means that the Download All button should work reliably on cached pages again.
You can update to version 1.5.2 from your WordPress plugin admin page. Here’s the full changelog for NextGEN Download Gallery.